More details have emerged about the man who allegedly went on a stabbing rampage in the Sydney CBD, which left one woman dead and another injured.

Mert Ney, 20, lived in a single-storey house with his mother and sister in Marayong, in Sydney's west and had recently been reported missing by his family.

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Police say Ney had some domestic violence issues linked back to his family that were being investigated.

"Police had taken a report for a domestic violence situation, so he was on the system (to) 'keep a look out for him'," NSW Police Commissioner Mick Fuller said on Wednesday.

Ney also had some issues around theft and malicious damage.

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But according to Fuller, Ney's criminal history was not considered too remarkable.

"He has some low-level issues around theft, malicious damage. He had some domestic violence issues linked back to his family, that again, were not significant or serious injuries," Fuller told reporters on Wednesday.

NSW Police Commissioner Mick Fuller speaks to the media alongside Minister for Police and Emergency Services David Elliott (left) during a press conference in Sydney, Tuesday, August 13, 2019. Credit: STEVEN SAPHORE / AAP

"But if you look at his spreadsheet from a criminal history perspective, it remains unremarkable.

"I've certainly seen kids at the PCYC behind me that probably have a worse spreadsheet ... than what we've seen with this individual."

Ney is in Blacktown Hospital undergoing a mental health assessment following his dramatic citizens arrest on Tuesday afternoon.

A special task force led by the homicide squad has been established to investigate the matter.

Chevron Right Icon 'I've certainly seen kids at the PCYC behind me that probably have a worse spreadsheet.'

Police will allege he killed a 24-year-old woman for no reason before stabbing another woman, aged 41, and threatening to kill a number of people at the same time.

A motive for the alleged attack remains unclear, and has left Ney's neighbours in shock.

Jeff, who lives in the same street, said he recognised Ney as soon as he saw the images on TV.

Neighbour Geff speaks to reports near the Sydney home of alleged CBD stabbing attacker Mert Ney. Credit: 7NEWS

"It's just a shock to have him as your neighbour - but you would not have expected anything," Jeff told reporters.

"We thought he was very quiet, (a) computer nerd quite frankly, just keeping to himself playing computer games.

"We had very little to do with him."

What we know so far