Six people were killed and several others injured -- including a nine-month-old baby -- when a knife-wielding attacker rampaged through a busy Sydney shopping centre on Saturday.

Australian police said multiple people were stabbed by the unidentified assailant, who was tracked down and shot dead by a policewoman who is being hailed as a national hero.

The incident occurred at the sprawling Westfield Bondi Junction mall complex, which was packed with thousands of Saturday afternoon shoppers.

New South Wales police commissioner Karen Webb said five women and one man had died. A baby was undergoing emergency surgery.

Police said the attacker is believed to be a 40-year-old man who was known to law enforcement, but he has not yet been formally identified. 

Webb played down suggestions that the attack could have been an act of terrorism and said it is believed the attacker acted alone.

"If it is in fact the person we believe it is, then... it's not a terrorism incident," she said. 

A New South Wales Ambulance spokesperson told AFP that eight patients were taken to various hospitals across Sydney, including the baby who was taken to the city's Children's Hospital. 

"They all have traumatic injuries," the official said.

Security camera footage showed a man wearing an Australian rugby league jersey running around the shopping centre with a large knife. 

Injured people lay lifeless on the floor, or surrounded by pools of blood. 

Eyewitnesses described a scene of panic, with shoppers scrambling to safety and police trying to secure the area.

Many people took shelter in shops, trying to protect themselves, their families and frightened strangers. 

Ayush Singh was working at a cafe inside the centre when the incident occurred. 

"I saw the whole thing in front of me," he told AFP. "I saw a lot of people running around, I saw the guy running with the knife and people running away."

Singh helped two elderly ladies who were having a coffee to hide inside his cafe. He heard three gunshots ring out, then saw the man lying on the ground. 

"It was really scary," he said. "I've felt really safe (in Australia). I've been here for six years. I didn't feel unsafe but now I feel scared." 

Prime Minister Anthony Albanese praised the bravery of strangers who helped each other and the woman police inspector who rushed headlong toward danger.

"She is certainly a hero. There is no doubt that she saved lives through her action," Albanese said. 

'Running and screaming'

Pranjul Bokaria had just finished up work and was doing some shopping when the stabbing occurred.

She ended up running to a nearby shop and taking shelter in a break room. 

"It was scary, there are some people who were emotionally vulnerable and crying," she told AFP.

She escaped using an emergency exit with other shoppers and staff, which took them to a back street.

She described a scene of "chaos", with people running and police swarming the area. 

"I am alive and grateful," she said.

As night fell, dozens of heavily armed police and ambulances were still outside the shopping complex, with stretchers ready to take people to nearby hospitals.

The sound of police sirens and helicopters filled the air. 

The mall has been locked down and police have urged people to avoid the area.

Britain's King Charles III said he and his wife Queen Camilla were "utterly shocked and horrified" by the stabbing. 

Pope Francis said he was "deeply saddened" by the attack and sent his "spiritual solidarity to all those affected" in a message addressed to the archbishop of Sydney.

Such attacks are virtually unheard of in Australia, which has relatively low rates of violent crime.