In the law, it's closely related to the doctrine of res judicata, which refers to a matter decided or adjudicated by a competent court and so conclusive between the parties to the matter.
The difference between the two, if there is one, is not often clear. I guess that functus officio might be used more for some discrete decision that occurs within a legal dispute, rather than the ultimate resolution of the case, but I reckon most folks involved in the justice system could think of many examples to shoot down such an imprecise attempt at a definition. Anyway...
edit: One of my colleagues helpfully pointed out the logical and simple distinction. Had I but spent more time looking...
In essence, res judicata refers to the dispute between the parties, and the rule that once decided, it can't be re-litigated.
Functus officio refers to the court, tribunal or office discharging or completing its role in the matter so that it can't re-enter the fray.
Last last year the Court of Appeal considered the issue in DPP v Edwards  VSCA 293.
Mr Edwards was sentenced by the County Court for recklessly causing a serious injury. We discussed a while back the changes which meant the County Court may not impose a suspended sentence for that offence, though the Magistrates' Court may.
The sentencing judge wasn't made aware of that, and so purported to impose a wholly suspended jail sentence on Mr Edwards.
A few weeks down the road, someone realised the problem, and ultimately, the County Court purported to re-sentence Mr Edwards to a community corrections order.
The Court relied upon Criminal Procedure Act 2009 s 412, which provides:
412. Power to amend when there is a defect or error
For the purpose of correcting any defect or error in substance or in form, a court may amend any summons, warrant, plea, judgment or order.
(That provision is similar to, but reads a bit broader, than Sentencing Act 1991 s 104A, also known as 'the slip rule'. We discussed the slip rule back here, and barrister Paul Duggan has just today published a new post on his blog, "Tripping up on the slip rule." Check it out.)
The DPP appealed the second sentence. The main point ultimately considered on the appeal centred on the validity of the second sentence.
One of the reasons the Court of Appeal seemed to be interested in dealing with that issue was because of its own previous decision in R v Brattoli  VR 446, and its apparent conflict with R v Billington  VR 265.
In Brattoli, the County Court first imposed a sentence that it could not properly impose, and then later purported to re-sentence the prisoner. (Well, pedantically, the second time was a sentence, rather than re-sentence.) The Court of Appeal said that was okay, because the original sentence was wholly invalid with the consequence the County Court had not at law imposed any sentence. So the second sentence was valid, and then exhausted or spent the sentencing power of the County Court.
(I know of one other case that dealt with a similar issue: Nollen v Police (2000) 78 SASR 421. That concerned a sentence imposed by a magistrate, later held to be beyond the jurisdiction of the Magistrates' Court and so unenforceable.)
In Billington, the Court of Appeal held that the trigger for the functus officio doctrine was when an order passed into "The Record". There are a few cases that deal with that, mostly suggesting it's when a judge's associate notes the order on the indictment (now presentment) or whatever the initiating process is, and the judge signs it. (I don't know of any cases that consider this point in the Magistrates' Court, especially now that most orders are recorded in CourtLink, the Court's computer system, rather than a paper register as seems contemplated by s 18 of the Magistrates' Court Act 1989.)
When is a court functus officio?
The practical problem that arises from these cases is that the functus officio rule tells us that once a court makes a decision, that's it, it's finished. Unless someone appeals, the decision stands.
But if the Court is not functus officio, the decision can be revisited.
In Edwards, Warren CJ nominated the divide as one between decisions of inferior and superior courts:
 The general position with respect to the finality of sentencing orders is clear. Once a court has made a sentencing order and the order has passed into the court’s records, the court is functus officio. The court cannot reconsider the matter, recall the original order and make a new order. This general position is supported by a long line of authority.
 This general position is subject to some qualifications, of which three are presently relevant. First, the court may have inherent or implied power to correct some kinds of errors in its orders (the ‘slip rule’). Secondly, statute may confer on the court additional power to vary a sentencing order. For example, statute may expand the court’s power to correct an error in the original order beyond the type of errors that can be corrected under the court’s inherent or implied power to correct errors. Statute may also confer power to vary a sentencing order in other circumstances. Thirdly, if the original sentencing order was made in excess of jurisdiction, it may lack sufficient legal effect to trigger the application of the functus officio doctrine. The existence of this third category is controversial.
 ...authority reveals a fundamental distinction between orders made by superior courts and orders made by inferior courts. Superior court orders are valid and effective until set aside, even if made in excess of jurisdiction. Generally, inferior cour orders are not.
What's a superior compared to inferior court? A quick and dirty ostensive definition is that all the state Supreme Courts and Courts of Appeal, the Federal Court and the High Court are superior courts. For a more expansive discussion, check out Enid Campbell, 'Inferior and superior courts and courts of record' (1997) 6 Journal of Judicial Administration 249. Some of the features of superior courts are: they posses supervisory jurisdiction exercised through prerogative remedies; their decisions are usually valid unless set aside, and so their jurisdiction is presumed to be valid; and they possess inherent jurisdiction. (Remember, inferior courts possess an implied rather than inherent jurisdiction — see Grassby v The Queen (1989) 168 CLR 1 — though some of the older cases don't make that distinction clear.)
The upshot of this is that if an inferior court makes an order in excess of its jurisdiction, that decision does not trigger the functus officio doctrine: Edwards at .
Warren CJ provided four reasons in support of this:
- This corresponds with the common law on administrative tribunals, which are similar to inferior courts.
- At common law, a party can disregard an order in excess of jurisdiction made by an inferior court, and not be liable to contempt. If the parties can disregard the invalid order, the Court should be able to as well.
- This is consistent with the position that decisions of inferior courts — in this case, the County Court — made beyond jurisdiction, are invalid, whereas decisions of superior courts are always valid until set aside.
- Brattoli is on point.
Her Honour considered that for all these reasons, the sentencing function remained "undischarged" and the County Court was not functus officio, at .
Weinberg JA and Williams AJA gave a joint judgment that took a different approach.
They too agreed that the first issue to consider was if the County Court was functus officio. At  –  they analysed a raft of cases, and said the answer was — the lawyers' favourite — "it depends". They adopted the reasoning of the High Court in Minister for Immigration and Multicultural Affairs v Bhardwaj (2002) 209 CLR 597 and subsequent cases, that the validity of a decision, of an inferior tribunal or court, made in excess of jurisdiction is determined by considering the legislation that provides the power to make the decision.
That meant in this case analysing County Court Act 1958 s 36A. Its wording and operation did not suggest that Parliament intended County Court orders to be void. So too, the existence of Sentencing Act 1991 s 104A and Criminal Procedure Act 2009 s 412 suggested that County Court orders should be considered valid and only perhaps voidable rather than void, if in excess of jurisdiction. Otherwise, what would be the need for those sections?
Their Honours held that on considering the source of the decision making power or powers available to the County Court, the Court was functus officio. Further, if Brattoli had ever been good law – and they doubted it had — it wasn't now.
The problem with functus officio versus void orders
When I read Warren CJ's decision, the first thing I wondered — as I had when I first read Brattoli and Nollen — was what happens when an inferior court imposes a sentence that is in excess of jurisdiction. If a magistrate purports to jail someone for an offence they can't impose jail for, or cancel a driver licence without power, is the offender free to just ignore that order? Who decides the order is invalid? The police? Lawyers? Offenders?
It seems to me there must be some certainty about those orders if we're not to have anarchy.
Their Honours dealt with that, providing (IMHO) compelling policy reasons why the doctrine of functus officio should be held to apply to decisions of inferior courts, even if made in excess of jurisdiction, until remedied on appeal.
 In DPP v TY (2009) 24 VR 705, 712, Maxwell P, Ashley and Neave JJA said that:
It is necessary that a court order imposing sentence be — and be treated as — valid and enforceable unless and until it is set aside (whether after a successful conviction appeal or after a successful sentence appeal). ... The status of court orders — at least those of superior courts — is quite different in this respect from that of administrative decisions.
 While their Honours did distinguish between inferior and superior courts (and so this statement of principle cannot be taken to have determined the effect of County Court orders made without jurisdiction), the case adverts to the strange consequences that would arise if a sentence infected by judicial error were to have no effect in law at all.
 Those consequences were specifically anticipated by Simpson J in Swansson (2007) 69 NSWLR 406, 432, where her Honour said:
In practical terms, of course, a person convicted (and imprisoned) as a consequence of a criminal proceeding that is a nullity is not able (even though entitled to do so) to disregard the order. But it might be useful to consider what would ensue if that person escaped from imprisonment. On the authority of Attorney-General (NSW) v Mayas and United Telecasters Sydney, he/she could not be convicted of escaping lawful custody.
 Any analysis requiring the result that a conviction for escaping lawful custody should be set aside on that basis would bring the law into disrepute.
 It would be somewhat odd to think that a judge of an inferior court had far greater powers to correct an error that he or she has made in sentencing an offender than a judge of a superior court of record.
In conclusion, the majority said:
 It is well established that intermediate appellate courts can hear appeals from ‘null’ convictions. At least to some degree, therefore, it can confidently be said that sentences passed in excess of jurisdiction are not, for all purposes, to be regarded as being without legal effect.
 In the absence of legislation, such as the provisions of the County Court Act 1958, the Sentencing Act 1991 and the Criminal Procedure Act 2009 to which we have referred, it would be plausible to suggest that a sentence imposed without power should be viewed as a nullity and without legal effect. Bhardwaj suggests, however, that labels such as ‘nullity’, or ‘void’, do not answer the true question as to the effects of a sentence imposed without power, which is one of statutory construction.
 In Victoria, having regard to the provisions of s 36A(2) of the County Court Act 1958, and the other provisions previously mentioned, a judge of the County Court is functus officio with regard to a sentence entered into the record of the court, even though it is affected by jurisdictional error. That conclusion flows from the weight of recent authority, including, in particular, the tenor of several recent High Court decisions. It also represents sound policy.
The end result
Their Honours went on to expressly overrule Brattoli. They noted that the County Court was functus officio following the first sentencing, and that the correct remedy was an appeal or to seek a writ of certiorari. The second sentence was invalid, because the Court was functus officio at the time.
Last, s 412 of the Criminal Procedure Act was not so broad as to cure all defects, and certainly could not overcome the effect of the functus officio doctrine.
This case is one of those text-book examples of how the common law gradually develops over time, and the courts refine the doctrinal approach to issues. Jettisoning Brattoli and the dual-stream of void-voidable decisions of inferior-superior courts, with the real chance for uncertainty, seems to be a much more logical result. It seems any order of any court will now be treated as valid and binding unless and until varied on appeal or review.