The son of the West Australian Police Commissioner has been sentenced to more than three years in prison for bashing his former partner 18 months ago.

Russell O'Callaghan was high on a combination of drugs when he repeatedly bashed and threatened to kill his former partner in front of their five-year-old son.

O'Callaghan, the son of Karl O'Callaghan, pleaded guilty to charges of assaulting and threatening the then 29-year-old woman at her home in August 2014.

Perth District Court heard O'Callaghan had gone to the woman's home to try to reconcile with her, but an argument started when she saw him sending text messages about drug-related matters.

Over the course of the next two days he assaulted her, including putting her in a headlock, choking her, sitting on top of her, holding scissors at her throat and striking her to the face, arms and back.

He made threats including that he was "going to bash the shit out of [her]", "gonna slit [her] throat", and said "if I can't be with you then I'm gonna kill you".

O'Callaghan's lawyer Sandra De Maio said her client's behaviour was the result of him taking a "staggering" amount of drugs including methylamphetamine, heroin and ecstasy.

"He was so drug affected the argument led to violence on his part," she said.

Ms De Maio said her client now accepted the relationship was over and he was committed to addressing his drug addiction.

She said he had the support of his father who "has assisted him, to see what options are available to him".

Karl O'Callaghan did not attend the sentencing hearing but provided the court with a reference for his son.

Russell O'Callaghan has been in custody on remand for 14 months, and his lawyer urged the judge to impose a suspended term.

But Judge Linda Petrusa ignored the plea, and sentenced O'Callaghan to 38 months in prison.

Judge Petrusa described the offences as "serious" saying they involved "protracted violence in a domestic setting".

She warned O'Callaghan he was at risk of reoffending if he did not address his drug addiction.

"Unless you get on top of your drug habit this isn't the last time you'll come before the courts," she said.

With time already served he will be eligible for release on parole in five months.

It is not the first time O'Callaghan has been in jail.

In September 2011, he was sentenced to 16 months in prison for his involvement in a clandestine drug laboratory that exploded.

Commissioner to stand by his son

Outside court, Ms De Maio said O'Callaghan had the support of his father.

"Absolutely most definitely 100 per cent support from any loving and caring father [and] that's never waivered," she said.

"Drugs affect kids wherever they come from however good your background is.

"If your child is on drugs it's an awfully difficult thing then to overcome and he's still trying to do that."

Commissioner O'Callaghan held a press conference hours after the sentencing to discuss his son's crimes, saying he was disappointed and distressed by the actions of the younger O'Callaghan.

"From my perspective I guess Russell's got this widely publicised methamphetamine addiction but methamphetamine use is not an excuse for family violence," he said.

"He didn't offer it as an excuse, I don't accept it as an excuse.

"I think we have to be very clear that the two things are distinct and other people in Western Australia can't expect to use usage of drugs as an excuse for crimes of violence against women."

The commissioner said he agreed with the sentence handed down by Judge Petrusa.

"This is a very serious offence, and the details are quite serious so there needs to be a response from the justice system which is appropriate," Mr O'Callaghan said.

"They have to send a strong message to people out there that family violence is unacceptable.

"So I think the outcome is appropriate."