Seven News journalist Robert Ovadia said the knifeman shouted "Allahu Akhbar" before police shot him, but had a history of mental health issues.
James Yeom, who was working at a nearby store, said he thought he saw two people shot by police in addition to the knifeman, who was shoeless and was carrying a kitchen knife.
"They just went down with the shot fire, I think they got shot too, they weren't able to get up," he said. "They appeared to be casualties of the shooting."
No one is believed to have died in the attacks but their conditions are said to range from "serious" to "critical", with wounds to their lower limbs and abdomens.
The knifeman reportedly has gunshot wounds to his right arm, his left thigh and abdomen.
Three people have been taken by road ambulance to Royal North Shore Hospital, while a fourth person was being airlifted there.
"A few of my customers came in saying they saw a guy holding a knife and they wanted me to call the police," Mr Yeong told Seven News. "When I was calling police I went out and was looking for this guy; he showed up in front of me holding a knife.
"I called police straight away. A few minutes later police officers showed up and asked him to drop the knife. He refused to drop the knife. They pointed their gun at him and asked to drop knife again, which he refused again. Then they shot him down.
"He looked really depressed. He wasn't looking too aggressive, he was just wandering around ...
"People realised he was holding a knife, looks like they were maybe calling police just like me, but a lot of people didn't realise what was happening. He was slowly moving around.
"A lot of people went past him (and) didn't realise he was holding a knife."
Mr Yeom said he believed three shots were fired and it appeared police were aiming for the man's legs. He said he thought two people were also shot in the melee, but this has not been confirmed.
"There were a lot of people here. I think all the people moved back to hospital or something but there are a lot of police officers in the area."
Bystander Kris Read said on Twitter that the man lunged at police with a knife before they shot him.
Nine News reported the man was seen wandering outside the mall looking "drug-induced and depressed" and was heard "babbling incoherently" just before the incident. Police sources said he was ranting.
The owner of the Blu Water Grill, directly adjacent from where the incident took place, could not give details of what he saw.
He told news.com.au his restaurant was in lockdown shortly after 12.30pm. "It's chaotic," he said.
The Daily Telegraph reported that four people had suffered injuries, including to their lower limbs and abdomen.
Sarah Henry, who arrived at the scene just five minutes after the shooting happened, told the newspaper: "I saw police over bodies, it looked like three people were on the ground and police and paramedics were working on them.
"You could see the blood on the ground, on the pavement.
"My husband saw the aftermath, saw people running away from the area.. He called me and said a shooting has happened.
"There were a lot of people standing around watching, it's happened in the market area outside St George Bank."
The knifeman has gunshot wounds to his right arm, his left thigh and abdomen.
NSW Police were on the scene quickly and the area was cordoned off and evacuated as shoppers fled for their lives.
A police spokesman said in a statement: "A critical incident has been launched after a police shooting in Hornsby today.
"About 11.50am emergency services were called to a shopping centre in Hornsby following reports that a man was armed with a knife.
"On arrival officers located the man and attempted to speak with him. Following a confrontation, a number of shots were fired by police.
"As a consequence four people have sustained various injuries and have been taken to hospital.
"A Critical Incident Team will now investigate all circumstances surrounding the incident, including the discharge of a police firearm.
"At this stage it is not known what has caused the confrontation. That investigation will be subject to independent review.
"No further details are available at this time however police are calling for witnesses to come forward.
"Police are not looking for any other offenders."
A NSW Ambulance spokesman confirmed to news.com.au that multiple ambulances were responding to an incident in Florence St, Hornsby, and they were treating multiple patients
Footage and images from the scene show the knifeman on the ground as others stand over him.
A customer service representative at Westfield Hornsby said the shopping centre was still open but certain areas had been cordoned off.
This is the second attack to take place in the location in the past three months.
On March 30, another major incident took place less than 100m away at the popular Kangnam Korean BBQ restaurant, also in Hornsby Mall.
On that occasion, 45-year-old Alexander Villaluna allegedly confronted a couple during their meal. He stabbed Jovi Pilapil, 38, and Keith Collins, 53, with a knife. Mr Collins, from the Central Coast, died at the scene.
Villaluna and Mr Pilapil were married. They separated and Ms Pilapil successfully applied to have an AVO in place against him months before the attack took place.
Witnesses said Villaluna, who was charged the following day with murder and attempted murder, calmly walked outside the restaurant covered in the couple's blood.
