Australian police have identified a 40-year-old man who suffered from mental illness as the perpetrator of a Sydney shopping centre stabbing rampage that killed six people and hospitalised 12 more.

New South Wales police Assistant Commissioner Anthony Cooke said Sunday the man had come from the northeast state of Queensland and was known to law enforcement.

"There is still to this point nothing that we have, no information we have received, no evidence we have recovered, no intelligence that we have gathered that would suggest that this was driven by any particular motivation, ideology or otherwise," said Cooke.

"We know that the offender in the matter suffered from, suffers from, mental health."

The 40-year-old man -- who was shot dead by a senior police officer at the scene Saturday -- was named as Joel Cauchi.

Five women and one man, a security guard, were killed in his attack, leading police to say they were investigating whether women were specifically targeted.

A Facebook profile said Cauchi came from Toowoomba, near Brisbane, and had attended a local high school and university.

A distinctive grey, red and yellow dragon tattoo on his right arm was used to help identify him.

He is believed to have travelled to Sydney about a month ago and hired a small storage unit in the city. It contained personal belongings, including a boogie board.

Saturday's attack took place in a bustling shopping centre in Sydney's Bondi Junction neighbourhood, not far from the famed beach.

Among those seriously wounded was a nine-month-old baby girl named Harriet who was said to be in a "serious but stable condition in hospital" with stabbing injuries.

- 'Beyond comprehension' -

The baby's mother Ashlee Good succumbed to her injuries after desperately passing the child to two strangers in the hope they could save the infant's life.

Good's family described her as "a beautiful mother, daughter, sister, partner, friend, all round outstanding human and so much more".

"To the two men who held and cared for our baby when Ashlee could not -- words cannot express our gratitude", they said in a statement to Australian media.

Two of the victims are said to have no family in Australia and attempts are being made to contact relatives overseas.

Prime Minister Anthony Albanese said Australians -- who are largely unaccustomed to incidents of violent crime -- were struggling to understand an "unspeakable" attack that is "really just beyond comprehension".

"People going about their Saturday afternoon shopping should be safe, shouldn't be at risk. But tragically, we saw a loss of life, and people will be grieving for loved ones today," he said.

"We also know there are many people still in hospital dealing with recovery, and our thoughts and prayers are with them."

Albanese said he had received messages from US President Joe Biden, British Prime Minister Rishi Sunak, and New Zealand Prime Minister Christopher Luxon among others.

He again hailed the officer, Amy Scott, who shot Cauchi dead as a hero.

The "wonderful inspector", he said "ran into danger by herself... without thinking about the risks to herself."

Outside the shopping centre early on Sunday, a collection of flowers started to build.

One white ribbon wrapped around a bunch of flowers simply said "RIP" with a heart drawn in black marker.

Families embraced as they lay flowers. One man stood silently and wiped away tears, before moving on.

Sydney resident Paul Hoolahan said he came to pay his respects to those who died at the shopping centre, where he often has coffee with his grandchildren.

"It is emotional," Hoolahan told AFP. "It shouldn't happen. It hit here," he said pointing to his chest.

New South Wales premier Chris Minns flew back from Japan on news of the attack.

At the scene on Sunday he said it had been "incredible to see complete strangers jump in, run towards the danger for their own lives in harm's way to save someone that they've never met before."

"We've got some wonderful people in our city," he said.