A "lone actor" who was known to police: What we know about the alleged Sydney stabber.

-With AAP.

UPDATE: The victim of the Sydney CBD attack has been identified as 24-year-old Michaela Dunn.

A 20-year-old man is expected to be charged after he allegedly killed a woman in an apartment, before he went on a stabbing rampage in Sydney’s CBD.

NSW Police Commissioner Mick Fuller said Mert Ney is expected to be charged with murder, as well as serious assault, after he unleashed a “terrifying carnage” in the city on Tuesday afternoon.

Ney allegedly stabbed a 41-year-old woman in the back at the Hotel CBD and then tried to stab countless others as he wandered the streets before being detained near Wynyard station.

The woman is in a stable condition in hospital.

The body of a woman was subsequently discovered at a Clarence Street apartment with Fuller stating the homicide and stabbing were linked.

Fuller said Ney was at the apartment for “business purposes”. It’s been reported the 24-year-old victim was a sex worker.

A lone actor.

Fuller says the rampage was “not currently classed as a terrorist incident” by authorities and the “lone actor” had no links to terrorist organisations.

The 20-year-old did, however, have “some ideologies in relation to terrorism” and the Joint Counter Terrorism Team will examine if the terrorism threat needs to be reassessed.

“There was certainly information found on him about other crimes of mass casualties and mass deaths around the world,” Fuller said, adding the material was discovered on a USB drive.

Video footage shows Ney, who was armed with a large butcher’s knife, yelling “Allahu Akbar” while wandering the CBD. He also jumped on top of a car and called on police to shoot him.

Known to police.

The Marayong man, who has spent the night under police guard in hospital, had a history of mental health issues.

Investigators are yet to speak to Ney, with NSW Police saying in a statement it was unclear "when he will be deemed medically fit to speak with investigators."

Fuller said it is hoped Ney will be released from hospital on Wednesday afternoon.

He was known to police but Fuller said "his history was unremarkable compared to the of gravity of the crimes he committed".

Police were reportedly already searching for Ney in relation to an alleged domestic violence incident involving his sister.

As reported by News Corp, Ney lived with his mother but had previously been homeless.

Fuller was asked about reports Ney may have absconded from a mental health facility last week.

"If I was forced to make a call... the evidence all points to that," he replied.

Ney walked out of Blacktown Hospital last Thursday morning after being treated for an overdose, news.com.au reported. Police were notified that he left, and his movements for the last few days are likely to form a major part of the police investigation.

He appeared at Blacktown Local Court in June after being caught with knuckledusters and was sentenced to a conditional release order with no recorded conviction.

Former classmates told the Daily Mail, Ney was a loner who kept to himself, but one said he had planned to attend a music festival last week.

"He just had a vibe, he was just an odd person - he didn't have many friends," he said.

"He was the weirdest guy who never said a word, not one, his head was always down in a book and in the breaks he would stand alone, never speaking to anyone," the former student said.

"He was a year above me but he stood out for being alone."

Neighbours told Nine News he was "quiet" and "kept to himself".

"Highest order heroes."

Three members of the public who gave chase and detained the attacker using chairs and a milk crate were described by the commissioner as "highest order heroes".

Superintendent Gavin Wood said the men's intervention had stopped other people from being hurt and described them as "significantly brave".

Fuller said while police weren't looking for any other offenders they couldn't discount coming across other crime scenes as the investigation continues.