Perth father Anthony Robert Harvey bought knives and planned murder of family, court told

GRAPHIC WARNING.

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.

Two-year-old twins Alice and Beatrix, three-year-old Charlotte and their mother Mara Lee Harvey, 41, were killed by Anthony Robert Harvey, 25, at their Bedford home on September 3, 2018.

He then murdered Beverley Ann Quinn, 73, when she visited the next morning, the West Australian Supreme Court heard today.

Ms Quinn and Ms Harvey were both struck on the head with a pipe and repeatedly stabbed using a knife he had bought 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 and notes on top.

"To my beautiful wife, I'm so sorry. I would give anything to undo what I've done," he wrote.

"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 even lied to his wife's employer to explain her absence from work.

Harvey had also sold items for money and took cash out of 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.

Prosecutor Amanda 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.

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.

The sentencing hearing continues.