(I say 'at risk' because the meaning applied to exceptional circumstances that make it unjust to restore a suspended sentence can vary widely: see R v Ioannou  VSCA 277 and contrast with DPP v Marell  VSC 430, for example).
What does punishable by imprisonment mean? The statute is ambiguous. To take a common example, s 49(2A) of the Road Safety Act 1986 provides that an offence under that section is punishable by different sanctions, depending on whether or not the offender has committed the offence before.
49. Offences involving alcohol or other drugs
(2A) A person who is guilty of an offence under paragraph (b), (f) or (g) of subsection (1), other than an accompanying driver offence, is liable-(a) in the case of a first offence, to a fine of not more than 20 penalty units; and
(b) in the case of a second offence-(i) to a fine of not more than 60 penalty units or to imprisonment for a term of not more than 6 months if the concentration of alcohol-
Is this offence punishable by imprisonment for the purposes of s 31 of the Sentencing Act 1991?
The answer (reasonably enough) is that the offence is punishable by imprisonment if a term of imprisonment was a sentence available to the Court. In the example above the offender would potentially breach their suspended sentence only if they had a prior offence that made their current offence a second offence.
The authority for this is Coleman v DPP & County Court of Victoria  VSCA 116.
Coleman was given a suspended sentence for fraud and criminal damage. He was brought before the Court again during the operational period of that suspended sentence for possession and use of cannabis. He pleaded guilty. The offence involved the possession and use of a small quantity of cannabis, offences which carry at most a fine. Under the same offence section, possession of a larger quantity, or the same amount with the intention of trafficking it, could result in imprisonment.
Vincent JA [at 20, Batt JA and O'Bryan AJA agreeing]:
Although there is no authority directly in point, the position is, I think, reasonably clear. As can be observed, it is not required in terms that the offender has committed a further offence in respect of which a sentence of imprisonment has been imposed. If that had been the legislative intention, a different form of words would almost certainly have been employed. "Punishable" as a matter of standard English language usage would ordinarily refer to the potential for punishment and not to its actual imposition. It is the commission of a "further offence" which attracts that potential penalty to which attention must be directed. Emphasis has been placed upon the consideration that, having been accorded the opportunity of avoiding incarceration, although the offender's earlier conduct was of such a seriousness that a sentence of imprisonment had to be imposed, there has been further criminal behaviour of a significant kind; the standard being the commission of an offence which carries the possibility of a sentence of imprisonment. Accordingly, as I see it, no question arises as to whether, in the individual case, incarceration would be appropriate in the proper exercise of sentencing discretion.
As earlier stated, the offence created by s.73 is punishable by imprisonment in circumstances where the offender fails to establish on the balance of probabilities that the mitigatory factors of a small quantity and possession for a purpose unconnected with trafficking are present. Their presence was not the subject of any dispute or serious doubt in the present matter. Could the appellant then be properly described as having committed "another offence punishable by imprisonment" within the meaning to be attributed to that expression in Section 31 of the Sentencing Act? In my view, the answer to the question must be - No. As a matter of basic common sense and fairness, to reach the conclusion that an individual had committed such an offence when there was not even a theoretical possibility that imprisonment could have been lawfully imposed appears to me to be absurd. The legislature is not to be taken to have intended to achieve such a result in the absence of a clear expression to this effect. The expression must, I consider, be read as a whole and be taken to incorporate any aggravating or mitigatory factors contained in the statute creating the offence upon which the possibility of a sentence of incarceration can depend.
In that case it was uncontentious that the drugs were for personal use. In cases involving cannabis where it is not so clear-cut, it may well be important for an accused to meet their onus under s 73(1) to avoid the restoration of a suspended sentence.