A serving police officer has been found guilty of unlawfully using a tracking device to monitor his mate's girlfriend.

Detective Sergeant Ian Gordon Tower, 42, installed the tracking device in the woman's car in September 2012.

The device was not police issue and the officer did not have a warrant to install it.

During a trial in the Perth Magistrates Court, Tower admitted installing the device but maintained he was acting in the course of his duty.

He said his 37-year-old police colleague, who cannot be named because of the nature of his work, had told him the woman was using drugs and he wanted to investigate who was supplying them.

Magistrate Richard Bayly rejected that suggestion, saying the story was not believable.

"That suggestion seems to me to be an unfounded slur on her character," Mr Bayly said.

Tower was found guilty of four charges but has been given a spent conviction.

He has been fined $4,000.

The woman's former boyfriend - who pleaded guilty to one charge of unlawfully using a tracking device - also avoided a conviction.

He was fined $3,000.

Tower has been stood aside from duties for the past year and the court has been told it is likely he will lose his job following the magistrate's finding.

The other officer is expected to keep his job.