Accused took selfie after alleged murder

As a young woman lay dying in her city apartment from stab wounds to her throat, the Sydney man accused of the horrific crime fled to a deserted city laneway and paused to take a selfie.

Mert Ney, 20, is then accused of running through Sydney's CBD, terrorising people with a knife yesterday afternoon and plunging it into the back of Lin Bo, 41, a Cosco worker who had been sitting in the Hotel CBD.

Before the alleged public rampage, Ney is accused of killing Michaela Dunn, 24, believed to be a sex worker.

The two had organised a meeting in Ms Dunn's apartment at 1.30pm but less than 20 minutes later police allege Ney had killed the young woman and fled.

In CCTV obtained by Seven, Ney was caught on camera exiting the building just before 2pm into a deserted city laneway.

Cameras caught Mr Ney casually walking into the alley, filled with rubbish bins and milk crates, before he can be seen holding up his phone and appearing to take a selfie.

The 20-year-old then looks briefly at his phone screen before walking slowly up to Clarence St where he breaks into a jog and starts his alleged rampage.

Heroic bystanders spotted Ney running around the city's streets with a butcher's knife about 2pm before they managed to detain him with chairs and a milk crate.

"Initial inquiries indicate the man had attended the unit for an appointment about 1.30pm and was captured on CCTV leaving the building about 1.50pm," NSW Police said in a statement today.

Ms Dunn's body wasn't found until 3pm, in the midst of the extensive police operation.

NSW Police Commissioner Mick Fuller said footage has been obtained of Ney in the city, "predominantly" CCTV from the City of Sydney council, in addition to the 20-year-old's phone and past records.

Officers have established Ney made phone contact ahead of an appointment with the 24-year-old victim, who was found dead in an apartment on Clarence St with injuries to her throat.

"We're rebuilding his movements," Mr Fuller told reporters today.

He said potentially the past two years of Ney's life will come under the microscope to aid what will be a long-term investigation.

Mr Fuller said it appears Ney recently self-admitted to hospital "following a medical issue" and there were subsequent mental health assessments.

The 20-year-old reportedly presented to the Blacktown Hospital emergency department on August 7 due to a drug overdose and was later discharged, according to The Daily Telegraph, but then absconded from an ambulance on August 8 while in the care of paramedics.

Ney is also believed to have been living in places other than his family's home in Marayong, in Sydney's west, in the days preceding his dramatic arrest.

"There's a property in Blacktown where we believe he was staying, so we're looking at that premises," Mr Fuller said, noting it was subject to a search warrant overnight.

"If he's been couch-surfing and homeless for some time, then we need to find every location that he's stayed in.

"We need to backtrack his history, not just the last 48 hours, but the last, potentially, two years."

Ney remains under police guard at Royal Prince Alfred Hospital and is yet to be charged.