A father wrote about embracing his “darkness and animal instincts” before murdering his three young children, wife and mother-in-law with newly purchased knives, a Perth court has heard.

The killings were so degrading, callous and violent that Director of Public Prosecutions Amanda Forrester told the West Australian Supreme Court on Friday that Anthony Robert Harvey, 25, should never be freed from prison.

“My position is that he should not have any hope of release – that this is a rare circumstance where the nature of the offending is such that that should be removed,” she said.

Two-year-old twins Alice and Beatrix, three-year-old Charlotte and their mother Mara Lee Harvey, 41, were killed at their Bedford home on September 3, 2018.

Beverley Ann Quinn, 73, was murdered when she visited the next morning.

Ms Quinn and Ms Harvey were both struck on the head with a pipe and repeatedly stabbed using a large hunting knife Harvey had purchased days earlier, which was almost the size of a machete.

Harvey had consumed wine and said he tried to “finish” his wife of three years as quickly as he could.

The children were murdered with a smaller knife as they slept because Harvey said “they were just little girls”.

Charlotte was stabbed 38 times, while Ms Harvey was stabbed at least 12 times.

The victims were covered by doonas, with flowers on top and toys around the children.

He also took photographs of them.

“To my beautiful wife, I’m so sorry. I would give anything to undo what I’ve done,” he wrote in a note.

“I think I’ve lost my mind.

“Take care of those little girls like you always do. I love you so much.”

Harvey slept after the killings and said when he woke up it felt like a nightmare, except nothing had changed.

He remained at the house for days and lied to his wife’s employer to explain her absence from work.

Harvey also sold items for money and took cash from his wife’s account before travelling about 1500km north to the Pilbara town of Pannawonica where he saw his parents.

“I’ve done something really wrong,” Harvey said.

“I hurt them … I miss them … It was Father’s Day the day before.”

With the help of his father, Harvey turned himself in to police on September 9.

He told authorities he had not been mad or angry, describing his marriage as good and his wife as fantastic.

Ms Quinn was also described as caring and he said he got along well with her.

Ms Forrester said it was a premeditated attack, noting Harvey purchased the weapons on August 23 and 29, and had written a journal entry about his plans on the day of the killings.

In the journal, Harvey wrote he must embrace his darkness and animal instincts.

“I am no psycho, I feel. I feel too much, I always have … I will regret what I do,” he wrote.

Ms Forrester said it was a gross breach of trust, describing the killings as brutal, callous and degrading.

She also noted the catastrophic effect of “wiping out” a family on the remaining extended family.

“It is inexplicable violence against people who were entitled to the offender’s protection (and) his love,” she said.

Defence counsel Sam Vandongen said his client was young and remorseful.

The court also heard details about Harvey’s mental health.

Justice Stephen Hall said he wanted to limit the continued suffering of the family but had matters he needed to consider.

Harvey will he sentenced on July 19.

-AAP