Two Perth police officers have been found guilty of assaulting a detainee by repeatedly tasering him.

Aaron Strahan and Troy Tomlin were senior constables when they tasered Kevin Spratt at the Perth Watch House in August 2008.

A magistrate found the duo's actions were not excused or authorised by law.

The men claimed Mr Spratt was going to attack them, but the magistrate rejected that argument.

Both officers were found guilty of three charges of assault, while Strahan was acquitted of a fourth charge.

The officers repeatedly tasered Mr Spratt after he refused to go to a cell to be strip-searched.

During the trial, prosecutors told the court there was no lawful basis for the officers to have used a weapon against the detainee and their actions were patently in breach of police guidelines not to use a Taser as a tool for compliance.

They described the officers' actions as inexplicable.

The court heard Mr Spratt had a history of violence and was under the influence of alcohol and possibly drugs, but prosecutors say there was nothing about his conduct which warranted the use of a Taser.

A witness, Sergeant Gary Thwaites, testified that before the detainee arrived at the watch house, Tomlin asked to be armed with a Taser because he had been told Mr Spratt had a history of violence.

Sergeant Thwaites told the court he believed under the circumstances the request was prudent and proactive, and he described Tomlin as a level-headed officer who was held in high regard.

Another officer, Senior Constable Emanual Bakovic told the court Mr Spratt was the strongest person he had come across in his life.

He testified that after Mr Spratt was tasered the first time, he became scared because the detainee was violent, aggressive, and kept fighting and twisting his body.

Convictions should reduce misuse of power: Spratt

Mr Spratt testified he had little recollection of the day and could only remember refusing a request to be strip-searched and throwing his ear-ring at a counter.

He said he did not remember being tasered.

Mr Spratt expressed his hope the verdict reduces the chances of further such incidents.

"It is a huge relief that justice has finally been delivered," he said in a statement.

"I am pleased that the court has confirmed no-one is above the law, and a Taser should only be used as a last resort.

"I am hopeful that today's convictions...will make it less likely others would suffer at the hands of police misusing their power."

Tomlin is now an auxiliary officer while Strahan is a sergeant.

Both were subject to internal disciplinary action by the police and were fined after being found guilty of unnecessary use of force.

The two men are due to be sentenced on Wednesday and prosecutors have argued they should face substantial fines.

However, their defence counsel says a good behaviour bond would be an appropriate sentence, and has also requested a spent conviction which means they would not get a criminal record.