The former clerk of WA's Upper House has been found guilty of four charges arising from his arrest near the home of his then boss whilst wearing a disguise and carrying a baseball bat.

Nigel Lake had argued in the Perth Magistrates Court he was not of sound mind when he was arrested in February 2015 near the western suburbs home of the clerk of the Legislative Council, Nigel Pratt.

But today the court ruled he was of sound mind at the time of his arrest.

Lake was on trial on charges of possessing an item with intent to cause fear, possessing a disguise, possessing a surveillance device and unlawful use of a tracking device.

Prosecutor Darren Renton told the court Lake pulled out a baseball bat when he was stopped by police about 11:30pm.

When police ordered him to lay on the ground they found he was wearing latex gloves, and a rolled-up balaclava.

Mr Renton said Lake identified himself as the deputy clerk of the Legislative Council and said he had been following his superior's work car because he believed he was "releasing confidential information".

He also said he had attached a tracking device to the wheel arch of Mr Pratt's car which was later found by police.

At the start of today's hearing Lake's lawyer, George Papamihail, said his client admitted the allegations against him, but maintained he was "insane" at the time.

Accused 'homeless, sleeping in car at Parliament'

The defence called psychiatrist Paul Skerritt, who told the court after assessing Lake in August this year, he concluded he was suffering from a "delusional disorder" at the time.

Professor Skerritt said the disorder had developed over about a year and at a time when he was having disputes with his wife, who also worked at State Parliament.

"He'd separated from his wife, was homeless and was sleeping in his car and office," he said.

He said Lake formed the impression he was being watched and subjected to surveillance, which prompted his actions in February 2015.

But prosecution psychiatrist Mark Hall, who assessed Lake in April 2015, said it was his opinion Lake had been suffering a depressive illness, but it had not deprived him of the capacity to know what he was doing was wrong.

In finding Lake guilty of all charges, Magistrate Michael Wheeler noted that Professor Skerritt had assessed Lake 18 months after the offences, while Dr Hall had seen him just a few months later.

Mr Wheeler said he "was far from satisfied" that Lake was affected by delusional beliefs at the time and therefore he was "still criminally responsible for the things he did".

"Eccentric behaviour is not equivalent to insane behaviour," he said.

Mr Papamihail said while his client had not been found to be delusional, he had a mental condition at the time.

He said Lake had lost a job that he took great pride in and had lost his reputation as a result of media reporting of the case.

Mr Wheeler fined Lake $2,300 and granted a spent conviction order, meaning he will not have a criminal record.