A senior police officer has been hailed as a hero for saving lives after she shot dead a man who fatally stabbed six people at a busy Sydney shopping centre.

The mass stabbing on Saturday afternoon at the Westfield centre at Bondi Junction has shocked the nation.

Six people, five women and one man, were killed in the attack and several others were injured.

Four female victims aged between 20 and 55 and a man, aged in his 30s, died at the scene. A fifth female victim, aged 38, died at St Vincent's hospital on Saturday night.

Local residents begin to lay flowers at the scene of the mass stabbing at Bondi Junction. – AAP

Also among those killed was Dawn Singleton, the 25-year-old daughter of multi-millionaire businessman John Singleton.

Two of the six victims were from overseas and have no family in Australia, NSW Police Assistant Commissioner Anthony Cooke said.

The male victim was working as a security guard at the shopping centre, NSW Police said.

The number of people in hospital rose by three overnight, taking the total to 12.

A nine-month-old baby was among those stabbed and underwent surgery on Saturday night, NSW Police Commissioner Karen Webb said.

NSW Health Minister Ryan Park said on Sunday the child remained in a critical condition in ICU at the Children's Hospital in Randwick. 

The child's mother, 38-year-old Ash Good, was one of those who died, passing her baby onto strangers and begging them to help before she lost consciousness. 

Police evacuated and sealed off the area following the incident. – AP

The attacker has been identified as 40-year-old Joel Cauchi, who moved to Sydney from Queensland last month.

Cauchi's family recognised him and contacted police after seeing news reports of the killings.

"The family when they viewed footage of the event on TV thought that may well have been their son and they reached out to authorities," said Queensland Police Assistant Commissioner Roger Lowe.

Cooke said the man was known to authorities in his home state. 

"We are and have spoken with his family and we will continue to do so and they are cooperating with us," he said. 

"We have received no evidence that we have recovered, no intelligence that we have gathered that would suggest that this was driven by any particular motivation, ideology or otherwise."

NSW Premier Chris Minns said many people ran "towards danger" including Inspector Amy Scott, who shot Cauchi dead at the scene.

"To the ordinary members of the public that cornered and confronted a murderer in the Westfield shopping centre, showing what I would call instinctive bravery under terrible circumstances," he said. 

"And of course Inspector Amy Scott, who ran towards danger and showed professionalism and bravery and without a shadow of a doubt, saved many, many lives in the last 24 hours.

"We're very, very grateful for the men and women that stood up in Bondi Junction."

Prime Minister Anthony Albanese said she was "certainly a hero" who no doubt saved lives.

"We have seen the footage of ordinary Australians putting themselves in harm's way in order to help their fellow citizens," Albanese said.

"That bravery was quite extraordinary … it's the best of Australians amidst this tragedy."

The shopping centre is closed and will be an active crime scene for days.

Minns said it was a truly shocking day in the state's history.

"I can understand that people will be grieving and very angry for the next few days," Minns said from Japan's Tokyo Airport, where he was waiting for a flight home.

"It's literally everyone's worst nightmare ... a wicked and a violent crime," he said.

Acting NSW Premier Penny Sharpe said a crisis cabinet had been convened following the tragedy and Minns had joined the meeting by phone from the airport in Tokyo.

Australian Federal Police Commissioner Reece Kershaw said the AFP was providing support and that ASIO and other agencies were involved in briefings.

Like all shootings involving NSW Police the investigation will be overseen by the Law Enforcement Conduct Commission.

Webb said Scott was doing well under the circumstances and would be interviewed on Sunday.

"She showed enormous courage and bravery," Webb said, adding other responding police, civilians and staff at the centre had too.

"It was an awful situation … but it could have been much worse," she said.

Inspector Scott was shown in local media footage administering CPR on the man after he was shot.

"When I met Amy last night ... we talked about her going straight into police mode, everything she has been taught during her career and how instinctive it was," NSW Police Minister Yasmin Catley said.

"When I said to her 'thank you for your courage' she said 'it was not just me, the bystanders around me were so helpful'. (She was) so humble, it's just typical of a NSW police officer."

Investigators inside the Bondi Junction shopping centre. – AP

Footage from inside the shopping centre showed people fleeing as a man armed with a knife chased them through the centre.

One man was seen in a stand-off with the attacker, holding a bollard at the top of an escalator and confronting the armed man as he approached.

The police inspector caught up with the attacker on level five of the centre as people fleeing gave directions, shooting him dead at the scene.

Video showed many ambulances and police cars around the shopping centre, and people streaming out.

Paramedics were treating patients at the scene. Police praised the bravery of bystanders who rushed to the aid of stab victims. 

“They just said run, run, run — someone’s been stabbed,” one witness said.

"(The attacker) was walking really calmly like he was having an ice cream in a park.

"And then he went up the escalators … and probably within about a minute we heard three gunshots.”

Witness Roi Huberman said he sheltered in a store during the incident.

"And suddenly we heard a shot or maybe two shots and we didn't know what to do," he said.

"Then the very capable person in the store took us to the back where it can be locked. She then locked the store and then she then let us through the back and now we are out."

Ayush Singh, 25, was working in a cafe in the mall when he saw the attack and then heard gunshots as police responded.

“I saw all the people running and I didn’t know what was happening," said Singh. “I thought it was some people playing a prank or something and after some time I saw a guy with a knife running from the footpath to the cafe where I work.”

Singh said he saw the man running just metres away as he wielded a knife. “I didn’t hear him say anything,” he added. "Just a random guy stabbing people. Mad guy.”

"As he walked just past beside me I heard two or three gunshots and the guy was neutralised," he said.

"People around me were terrified. There were some old ladies I helped to get them inside a safe place inside the cafe."

Reese Colmenares describes seeing the injured infant being evacuated by ambulance. – AP

Reese Colmenares, an eyewitness who hid in hardware store Mitre 10 with 20 others when people started running out of the mall, said she saw a baby with stab wounds being taken to an ambulance.

“The mother was terrified, the mother was sad, just holding (and) comforting the baby," she said.

“Even 20 minutes after people were rushed out of the mall, I saw SWAT teams of people sweeping the surrounding streets,” one witness said.

The other said they saw a woman lying on the ground and took shelter in a jewellery store.

An eyewitness described the police officer shooting the attacker.

"If she did not shoot him, he would have kept going, he was on the rampage," said the man, who did not give his name. "She went over and was giving him CPR. He had a big blade on him. He looked like he was on a killing spree."

Other witnesses said the man was wearing shorts and an Australian national rugby league jersey, and looked confused and appeared to attack randomly.

“He was not looking for anyone personally,” one witness said.

“He was just running around with his knife.”

One video showed a man confronting the attacker with a bollard from the top of an escalator. Others showed people trying to move the injured to safety and help others hide in shops.

"I just saw the footage of the guy with the bollard, I reckon he’s the real hero here,” Bondi Rescue lifeguard Andy Reid, who was also shopping at the mall, said. Reid also attended to a woman who was bleeding on the fourth floor.

Several posts on social media showed crowds fleeing the mall and police cars and emergency services rushing to the area.

A history of violent crimes in Australia

Such violent crimes are rare in Australia, a country of about 26 million people, which implemented one of the world's toughest gun laws more than two decades ago.

Here is a list of some of the country's major violent attacks.

April 1996 Lone gunman Martin Bryant went on a shooting spree at a cafe and tourist site at the former colonial prison of Port Arthur in the island state of Tasmania, killing 35 people. He used military-style weapons that he had bought without background checks. The incident prompted Australia to implement some of the world's toughest gun laws.

December 2014 Three people, including attacker Man Haron Monis, were killed when police stormed a cafe in Sydney's Martin Place to free hostages held at gunpoint for 16 hours. Monis, a self-styled sheikh from Iran who received political asylum in 2001, was well known to Australian authorities, having been charged as an accessory to murder and other crimes.

January 2017 A man deliberately drove into pedestrians, killing five and injuring more than 20, in Melbourne. Police said the incident was not terrorism-related. The 26-year-old had a history of family violence. Authorities later installed 140 concrete posts in the city centre.

December 2017 Afghan former refugee Saeed Noori, 32, deliberately sped up and drove into dozens of pedestrians crossing the road at one of the busiest intersections in Melbourne's central business district, injuring 19. Police said they did not believe the incident was terror-related.

November 2018 An attacker in Melbourne was shot by police after he set fire to a pickup truck laden with gas cylinders in the city centre and stabbed three people, killing one. The attacker died in hospital. Islamic State claimed responsibility for the attack.