Moments before a Perth dad killed his two children, he made a "sick" call to his wife.

Warning: This post includes details of a murder case that might be distressing for some readers.

A father who wanted to punish his wife for leaving him used their young children as “weapons” to “break her heart into 50 million little pieces” by callously killing them, a West Australian court has heard this week.

Police found the bodies of five-year-old Zaraiyah-Lily Headland and three-year-old Andreas Headland inside a Yanchep home on October 20 last year. The children’s father, 36-year-old Jason Craig Headland, was also found at the scene with self-inflicted stab wounds, including three to his neck.

On Thursday, the West Australian Supreme Court heard that the cause of death could not be ascertained but the children had been drugged with sleeping tablets that were crushed up in their juice and then later asphyxiated.

Justice Lindy Jenkins described Headland's actions as vindictive and the most extreme act of selfishness.

"You had a duty to love and protect them. Instead of protecting them, you used them as weapons," she said.

"You made your children pay with their lives in order to inflict pain and loss on your wife."

The court heard Headland's relationship with his wife Anatoria had deteriorated over a number of months, and after she demanded they take a break Headland decided that he wanted to take the children away from her.

"I'm going to break your heart into 50 million little pieces," Headland told her over the phone moments before he killed the children, adding it would be the last time she would speak to them.

Anatoria also heard Headland giggling before he hung up.

In a note found at the house, Headland told his wife she would have to "live with this for the rest of your miserable life".

Justice Jenkins described the note as "callous, shallow and self-centred".

In her victim impact statement, Anatoria said that her children were bubbly and excited to learn, describing them as her heart and soul.

"Two peas in a pod, they were, and polar opposites," she said. "They completed each other and they completed me."

Anatoria described her daughter as a "determined little girl" and her son as a "brave soldier".

Since their murders, she has suffered anxiety and depression, and is scared that if Headland is released he will come after her.

"You make me sick. I'll never forgive you," she said, addressing Headland in her statement on Thursday. "What you did was heartless and selfish."

Headland's defence lawyer argued that the 36-year-old had a strong sense of family values and was an active member of the indigenous community, including being a mentor to young Aboriginal men. He also claimed Headland had no memory of what happened on that October day.

Prosecutor Amanda Forrester argued Headland had shown no remorse or contrition in the months that have passed, describing him as self-centred and lacking insight or empathy.

He may have been a loving parent, but he "destroyed every remnant of that" with his "grotesque act of family violence", she said.

"There is no such thing as a comparable case."

Headland was sentenced to life behind bars, with a minimum of 31 years to be served.

If you or someone you know is in need of help, please call the National Sexual Assault, Domestic and Family Violence Counselling Service on 1800 RESPECT.