A selfie picture of a person in blonde hard and big dark sunglasses

Triple killer Regina Arthurell, 79, was found to be living with children. (Supplied)

A NSW government department had known since December that two foster children were living under the same roof as a triple killer in Western Sydney, but did not address the situation until this week.

Minister for Families and Communities Kate Washington made the admission on radio station 2GB, saying the mistake was the worst she had ever seen during her time in the portfolio.

Ms Washington said it was “entirely unacceptable”.

“It should never have happened, and I’m deeply apologetic for what has happened,” she told host Ben Fordham.

A listener earlier alerted the radio station that a 12-and 14-year-old had been living in the same home as 79-year-old Regina Arthurell.

The triple killer was previously known as Reginald Arthurell.

In 1995, Arthurell murdered a former partner by beating her in the back of the head with a piece of wood.

Arthurell had already been convicted of manslaughter on two previous occasions.

While Ms Washington only became aware of the matter in recent days, she said her department had known for months.

“The department was made aware of the situation in late December, and some very poor decisions were made at that time,” she said.

“I wish I could turn back the clock, but I can’t,”

she said.

Kate Washington wearing a red jacket

Kate Washington says she personally only became aware of the matter in recent days. (ABC News: Timothy Ailwood)

Investigation launched

Ms Washington said the Department of Communities and Justice had launched a review.

“This review is going to go through the whole timeline,” she said.

“Who knew what, when? And who made what decisions, when?

“Because obviously it is very clear that very poor decisions were made through this process.”

The minister said Arthurell was no longer living with the children.

Asked whether she had ever seen a mistake this big in her portfolio, Ms Washington replied: “No, I haven’t. I could not have been more shocked and horrified.”

Supervision order not extended

In 2021, after Arthurell’s parole expired, an extended supervision order was imposed, including conditions such as electronic monitoring and alcohol rehabilitation.

At the time, Justice Richard Button noted Arthurell was not in good health and had made a “sincere effort” at rehabilitation.

However, he said Arthurell had “unequivocally demonstrated a proclivity to violently terminate the lives of fellow human beings”.

“A frail, even physically disabled, person can inflict fatal harm once armed with a weapon,” he noted.

Up close of Michael Daley in a black suit and navy tie.

Michael Daley was asked during a budget estimates hearing why Arthurell’s supervision order was not extended. (AAP: Bianca de Marchi)

In a budget estimates hearing on Wednesday, Attorney-General Michael Daley was asked why Arthurell’s supervision order was not extended when it expired in 2024.

“If Arthurell does not meet the threshold for you to apply for an [extended supervision order], who does?” Shadow Attorney-General Damien Tudehope asked.

While Mr Daley took on notice some questions regarding Arthurell’s case, he said a High Risk Offenders Assessment Committee provided advice to the government on extended supervision orders.

“There are months and months of medical and psychiatric and other assessments made by the experts that underpin that panel,” Mr Daley said.

“And attorneys-general act on advice in relation to those matters.”