The police saw Mr Farmer sitting in a car parked in a bicycle lane. The headlights were on. Just after the police saw the car, the headlights went off. The police stopped behind the car and walked up to it. Mr Farmer was talking on a mobile phone. He was wearing his seatbelt; the keys were in the ignition; the engine was running.
The police asked him to take a preliminary breath test. Mr Farmer refused.
He was charged with that offence, contrary to Road Safety Act s 49(1)(c).
The magistrate dismissed the charge, saying:
I am not satisfied beyond reasonable doubt that the informant’s belief was, in fact, held on reasonable grounds in this case and on that basis the charge is dismissed.
The police can do a preliminary breath test on a driver they find driving or in charge of a motor vehicle.
Section 3AA of the Road Safety Act provides for a person in-charge of a motor vehicle.
3AA. Circumstances in which person is to be taken to be in charge of a motor vehicle
(1) Without limiting the circumstances in which a person is in charge of a motor vehicle, the following persons are to be taken to be in charge of a motor vehicle for the purposes of this Act—
(a) a person who is attempting to start or drive the motor vehicle;
(b) a person with respect to whom there are reasonable grounds for the belief that he or she intends to start or drive the motor vehicle;
(c) a commercial driving instructor while the person whom he or she is teaching to drive is driving or in charge of the vehicle;
(d) an accompanying licensed driver while the person whom he or she is sitting beside is driving or in charge of the vehicle.
The police alleged they reasonably believed he was in-charge of the motor vehicle, so they were entitled to require a PBT.
The Supreme Court covered the main cases dealing with reasonable belief compared to suspicion, at  – . The Court also emphasised at  and  –  that the Magistrate's role was to consider if the police informant's belief was reasonable, rather than if he (the Magistrate) would hold the same belief.
The prosecution appeal succeeded, because the Magistrate applied the wrong test — holding that belief was a state of mind approaching certainty — and considered he had to reasonably believe the accused intended to drive.
This case referred to a previous decision of DPP v Mitchell (2002) 37 MVR 142, on reasonable belief of police officers for drink-driving allegations, but curiously not to the Court of Appeal judgment affirming that decision in DPP v Mitchell (2004) 40 MVR 358.
There are a few older cases on in-charge, before s 3AA was introduced. Woods v Gamble (1991) 13 MVR 153 held that the intent to drive must be immediate (or just about). Davies v Waldron  VR 449 said that starting a car could be enough to be in-charge. Neither case was mentioned in DPP v Farmer, but nothing in this case is inconsistent with them, so it seems they're still good law.