The father of Bondi Junction stabber Joel Cauchi says his son was a “very sick boy” who had a fixation with knives and may have been targeting women because he struggled to get a girlfriend.

Speaking to media outside his Toowoomba home on Monday, Andrew Cauchi said the 40-year-old killer, who had schizophrenia, had committed something “so horrendous that I can’t even explain it”.

“He was a tormented soul, tormented, and frustrated, and I’m sorry that he’s done this to your children and this nation,” he said.

“There’s nothing I can say, there is nothing I can say that will take away the pain that my son has caused.”

Asked if he knew why his son may have targeted women, as police have suggested, Cauchi said: “Yes, because he wanted a girlfriend and he has no social skills and he was frustrated out of his brain.”

Five women and one man were murdered in the stabbing rampage at the busy shopping centre.

The assailant’s mother Michele said her boy may have been having a psychotic episode when he killed six people at Westfield Bondi Junction on Saturday afternoon.

“This is a parent’s absolute nightmare when they have a child with mental illness, that something like this would happen, and my heart goes out to the people my son has hurt,” she told the ABC.

“If he was in his right mind he would be absolutely devastated at what he has done, but he obviously was not in his right mind, he had been triggered into some kind of psychosis and lost touch with reality.”

The Toowoomba couple was watching the unfolding event on Saturday and suspected the man they saw in the clips, wearing a Kangaroos jersey and gripping a knife, was their son.

“My wife said ‘This looks like Joel’, when you saw him on the stairway with his head down, and I said it might look like Joel but I’m not going to say it’s Joel,” Cauchi said.

“I watched and watched, and I watched until 3 o’clock in the morning.

“The police then knocked on my door that night to tell us they believe this is Joel, and I said ‘you don’t have to believe it, you can know it’.”

Cauchi told the ABC’s 7.30 program Joel, who was plagued by mental illness, lived at home until the age of 35. He then moved to Brisbane and stopped seeing his regular doctor.

Cauchi said in January last year his son moved back to Toowoomba and brought several US Army combat knives. His father took them from him, saying he did not want the knives in his house.

“He rang the police saying I stole his knives … I said ‘look I am not having these in my home, I have given them to a mate to look after them’,” said Cauchi.

“I told the police, ‘My son had schizophrenia and I’m worried for myself’. I said to my mate: ‘Why do I feel I’m going to be killed in my own house by my own son with a US combat knife?’.

“The reason I said that was because I knew what people with a fascination with weapons and have mental illness – which my son has had since he was 17 – can do.”

NSW police are heading to Queensland to try to unravel Joel Cauchi’s motives.

Eight victims remain in hospital, including an infant girl in intensive care.

Cauchi was shot dead by police at the scene, after seemingly singling out women in Australia’s worst massacre in recent years.

Commissioner Karen Webb said late on Monday NSW police would go to Queensland to talk to Cauchi’s family, associates and friends to “give us an insight into the offender and what he might have been thinking”.

“He had a fascination with knives, we’ve been told that. What else was on his mind we really don’t know at this stage,” she told radio station 2GB.

Tweet from @2GB873

As a floral tribute to the victims grew at Bondi Junction, NSW Premier Chris Minns said a special coronial inquiry would examine the circumstances of Cauchi’s “horrifying, vile act”.

The inquiry, bolstered by up to $18 million in extra funding, would look at the police response, as well as the killer’s interactions with NSW and Queensland agencies.

Minns said the killer’s motive might never be known, but it was “the truth of the matter” that women were targeted. The rampage may also pave the way for security guards in shopping centres to be armed.

The killer’s father Andrew Cauchi said his son, with whom he had periodic contact, battled mental health issues, such as schizophrenia, for decades.

“I’m loving a monster. To you he’s a monster, but to me he was a very sick boy,” Cauchi said outside his home in the regional Queensland city of Toowoomba.

Hundreds of people were fled the eastern Sydney shopping centre during the attack that claimed the life of 38-year-old osteopath Ashlee Good and Dawn Singleton, 25, the daughter of high-profile businessman John Singleton.

Architect Jade Young, 47, artist and designer Pikria Darchia, 55, and 27-year-old Chinese student Yixuan Cheng were also murdered.

Yixuan Cheng was named as the sixth and final victim of the Bondi stabbings. Photo: TV screen grab

Security guard Faraz Tahir, 30, a Pakistani refugee, was the only man killed.

The terrifying attack, vision of which was widely circulated on social media, has led to an outpouring of grief in the state where such killings are rare.

Prime Minister Anthony Albanese, who laid flowers at the scene on Sunday, said he had spoken to the families of several of those attacked.

“The gender breakdown is, of course, concerning,” Albanese said.

Since the attack, official flags have flown at half mast and the Sydney Opera House sails will be lit with a black ribbon on Monday night.

A permanent memorial is being considered for near the Bondi shopping centre site.

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-with AAP