A small evening vigil was held Sunday for the six people killed in a knife attack at a busy Sydney shopping center, which police said was carried out by a local man with a history of mental illness.

Mourners gathered in silent reflection outside the Westfield mall in Bondi Junction, which had been packed with weekend shoppers when 40-year-old itinerant Joel Cauchi went on a stabbing rampage on Saturday.

Police said five women and a Pakistani security guard were killed in the attack, which lasted for about half an hour, until a solo policewoman tracked down Cauchi and shot him dead.

Inspector Amy Scott was hailed by Prime Minister Anthony Albanese as a "hero" who "no doubt" had "saved lives through her action".

Among Cauchi's victims were a designer, a volunteer surf lifesaver, the daughter of an entrepreneur, and a new mother whose nine-month-old baby is still in hospital with serious stab wounds.

As night fell Sunday, a group of about 40 people from a local Muslim association placed flowers on the ever-growing pile outside the shopping center.

They remembered 30-year-old Faraz Tahir, who had been working as a security guard when he was stabbed.    

They stood for a minute of silence with their hands clasped, heads bowed, and eyes fixated on the flowers.

The mourners then raised their hands in a moment of prayer.

Many wiped away tears.

Australians, largely unaccustomed to violent crime, are still coming to terms with an attack that shattered a city better known for its famed beaches and laid-back bars and restaurants.

On any given weekend the Westfield shopping center is packed with people shopping for clothes or groceries, with families grabbing a bite to eat or a movie.

Health officials said it would take many eyewitnesses a lifetime to come to terms with what they saw and felt.

"The sound of people screaming was horrific," said one eyewitness, Daphi Kiselstein, who was shopping at the time of the attack and took refuge in a store with other terrified shoppers.

Despite early social media reports falsely linking the attack to events in the Middle East, New South Wales police Assistant Commissioner Anthony Cooke said there was no evidence that Cauchi was "driven by any particular motivation, ideology or otherwise".

Police said he was diagnosed with a mental health issue at age 17.

'Only doing her job'

In a pained statement, Cauchi's parents offered thoughts for the victims and said their son's actions were "truly horrific".

"We are still trying to comprehend what has happened. He has battled with mental health issues since he was a teenager."

The parents also sent a message to the officer who shot their son dead.

"She was only doing her job to protect others and we hope she is coping alright," they said.

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

He had been living in a vehicle and hostels, and was only in sporadic contact with his family via text messages, his parents said.

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.

'Outstanding human'

One victim, 38-year-old mother Ashlee Good, succumbed to her injuries after desperately passing her bleeding baby to two strangers in the hope they could save the child'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.

The baby, named Harriet, was said to be recovering well after lengthy surgery.

Prime Minister Albanese said Australians 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.

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

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.