A West Australian man convicted of bludgeoning an elderly woman to death with a tomahawk and robbing her has been sentenced to life in prison.

Colin Peter Casey was sentenced in the Supreme Court on Tuesday for the murder of 73-year-old grandmother Helen Bromley at her Mandurah home, south of Perth, on October 8, 2010.

Prosecutor Amanda Burrows said Casey went to Ms Bromley's home intending to steal money for drugs and when she confronted him on her patio, he panicked and picked up a tomahawk that was nearby.

When Ms Bromley told him she was going to call the police and turned to go back into the house, Casey struck her twice on the head and punctured her skull, causing blood loss and exposing her brain.

He then covered her body and stole some jewellery before fleeing, Ms Burrows said.

Casey's lawyer Mara Barone argued that while it was a violent offence against a defenceless woman, the attack was unplanned and Casey had had no desire to hurt her.

Ms Barone said the attack was a "callous overreaction" after Ms Bromley said she was going to call the police.

He had not struck her once she was down, she said.

"It was done not with any clear thought at all," she said. "It as an impulsive and panicked decision."

Ms Barone said her client was a functioning drug user until he started taking amphetamines and his "life fell to tatters".

Reading from a report, Ms Barone quoted Casey as saying he accepted that he had to be imprisoned for taking someone else's life and apologised to Ms Bromley's family.

"I can't even imagine what they are going through," he said. "I don't know how to say I'm any more sorry.

"I took a life so I deserve to give mine."

In her submissions, Ms Burrows said although Casey did not have a violent criminal history, he did have 44 convictions against him.

"It shows a continuing disregard for the law," she said.

In a victim impact statement, Ms Bromley's daughter said she felt separated from the world since her mother's murder.

"We've been in limbo for so long ... the nightmare never stops," she said.

"My family should never have been put through all this."

Casey was arrested on September 2, 2011 and has been in custody since then.

While Justice Eric Heenan sentenced Casey to life in prison, he deferred setting a minimum term until February 15.

Any sentence for the aggravated burglary charge will be served concurrently with the murder sentence.

AAP anr/rlm/w