Sydney stabbing suspect's sister breaks her silence after rampage

The sister of a man accused of slashing a woman to death and stabbing another in a Sydney CBD rampage has apologised for his actions, but says talk of extremist ideology is "news to her".

Investigators are trawling through Mert Ney's electronic devices, social media accounts and family home as they try to determine why the 21-year-old sought to kill and injure innocent people on Tuesday.

Ney remains under police guard in hospital and is yet to be interviewed by police about the incident that brought the city's streets to a standstill.

Mert Ney, 21, is arrested after his alleged rampage. Source: AAP

NSW Police Commissioner Mick Fuller said the rampage had not been classed as a terrorist incident and the "lone actor" - who is likely to be charged on Wednesday - had no known links to terrorist organisations.

Mr Fuller also said Ney had recently self-admitted to a mental health facility after experiencing problems, and had been listed as missing by family.

Ney's sister on Wednesday told reporters outside the family's Marayong home in western Sydney that Ney had been domestically violent in the lead-up to the incident and suffered several mental illnesses.

But she didn't believe Ney subscribed to extremist ideology.

Mert Ney's sister said she's "really sorry" for her brother's alleged actions. Source: Nine News

Police say Ney was carrying a USB stick with information about recent mass casualties in New Zealand and the United States, and could be heard on video footage shouting "Allahu akbar".

"All the words I can say won't ever bring her back. I want to say I'm really sorry, she was defenceless," said Ney's sister, who asked not to be named.

"All this stuff about extremism, white supremacy, stuff like that, that's news to me ... it was using religion as an excuse."

However mental illness will not come into consideration if the attack is deemed an act of terrorism, which could entail significantly stronger punishment.

Ney's restrained by members of the public. Source: Brodie Smith/PA Wire via AAP

"There's this debate worldwide around terrorists and mental health, and for mine, they don't join - at the end of the day there's rules around me for declaring a terrorist incident," Mr Fuller told reporters.

"The mental health aspect is a distant second for me at the moment."

Police believe Ney attended a unit on Clarence Street for "the purposes of prostitution", where the body of 24-year-old Michaela Dunn was later discovered.

Ms Dunn, from Sydney's inner west, had a laceration to her neck and was pronounced dead at the scene.

Ney was captured on CCTV arriving at the building at 1.30pm and leaving about 20 minutes later, before allegedly stabbing a 41-year-old woman in the shoulder at Hotel CBD.

Michaela Dunn, 24, who died from stab wounds. Source: NSW Police

AAP understands the woman is Linda Bo, an employee at shipping company Cosco, which has an office nearby. She is in a stable condition in hospital.

"You would say with some confidence that he went on the street to kill more people, and the fact she's alive is a miracle," Mr Fuller said.

He then walked down York Street, armed with a large kitchen knife, before being restrained with chairs and a milk crate by bystanders.

Ney has undergone surgery at Royal Prince Alfred Hospital for a cut to his knee but police hope he will leave hospital on Wednesday afternoon. He is expected to be charged with murder, as well as serious assault.

The commissioner again praised the members of the public who helped restrain the 20-year-old, labelling them "true heroes".

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