Carl Robert Trevenar had maintained his innocence at a two-day hearing in June, denying he committed theft when he removed a tub of liquid fertiliser from the Nowra Police exhibits room.

But on Tuesday magistrate Gabriel Fleming found him guilty of stealing on the job and of a proceeds of crime offence stemming from RMS inquiries into his ownership of a seven-metre trailer.

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The court heard Trevenar had operated a kayaking business in South Nowra while also working as a police officer, and that he once used a small boat trailer, described as a "tinny" to advertise the business.

The court heard he paid his parents' Corrimal neighbour $2000 in 2014 for a seven-metre, fully-enclosed, unregistered trailer, after it appeared in the man's driveway.

The neighbour claimed in court to have tried in vain to find the trailer's owner.

Trevenar put number plates from the since disposed-of "tinny" onto the bigger trailer, and paid the registration from 2014 until 2019, when an RMS inspector noticed the trailer was significantly bigger than the one registered to that number plate.

Magistrate Fleming found many aspects of Trevenar's evidence surround the trailer "vague and at times implausible".

"There is no dispute that he was a police officer of 30 years' standing. He was aware of the lack of ID compliance plates [on the 7m trailer], and that this could indicate the vehicle was stolen, or had been the subject of fraud," she said.

"He went to considerable lengths to misrepresent the registration of the trailer, by placing the ... "tinny" plate on it for a considerable period. His evidence that he had three other vehicles and four trailers and didn't remember the details was not credible."

The court heard Trevenar was in a state of personal distress at the time, due to a relationship breakdown.

"While I accept the evidence of his personal situation, I don't accept the defendant was thus incapable of carrying out his legal obligations in relation to the registration of the trailer," Magistrate Fleming said.

"This is just implausible ... it's incongruous with him continuing his work as a NSW Police officer, indeed in a position where attention to record keeping was paramount - as the exhibits officer in a large and busy police station, recording and registering items that were confiscated by police."

"This was not simply a matter ... where there was a passive failure to register the trailer. it was an active registration ... for a trailer that he knew did not exist."

"The defendant's conduct in relation to the trailer clearly shows consciousness of guilt and the suspicion that the trailer indeed may be the proceeds of crime."

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Trevenar was also found to have taken a 25-litre container of "Monsta Bud" fertiliser from the Nowra police exhibits room in September 2014, after falsely marking the exhibit as "internal disposal - destroyed".

The fertiliser, worth $230, had been seized during a drug raid. It was still bearing an exhibits bar code when it was discovered at his property in 2018.

"There are clear, strict and documented procedures for the handling of police exhibits. He was well aware of these, this was his job," Magistrate Fleming said. "To suggest that an informal conversation with another officer about what he had done in the removal of the Monsta Bud was some implied approval of that is also not credible ... that is contrary to everything we know about the handling of evidence."

The magistrate accepted the evidence of Trevenar's then-partner, who told the court he brought the fertiliser home "and told her that he intended to use it on the plants because he believed it would be otherwise thrown away".

Trevenar was found not guilty of a further charge of dishonestly obtaining the trailer by deception.

The matter returns to court in December for sentencing.

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