The body of a young woman found in an alley cannot be removed while a police operation to coax a man off a Sydney apartment rooftop continues.

The body of the woman, believed to be in her 30s, is covered by a blanket, but has been lying out in the sun, with Sydney temperatures reaching a high of 27 degrees.

Earlier today a pedestrian made the grisly discovery of the woman's body in an alleyway at about 6.30am on Brown Street near Chatswood train station.

The woman appears to have fallen from somewhere in the Brown Street apartment building.

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Police and a Korean interpreter have been speaking to the man who is dangling above an awning over a communal gym area on the 26th floor of the building.

At one point this afternoon he leaned his head right over the awning.

Police have said they cannot remove the victim's body and secure the crime scene while the man is still on the awning.

A police spokeswoman at the scene confirmed officers believed they knew who the man was, but provided no other details.

The man who discovered the woman's body told the Daily Telegraph he thought the woman was asleep when he found her.

"I went over to wake her up, but then realised she was not going to wake up," Luke Bladen Williams Hamilton said.

"There was blood coming out of her face."

Mr Hamilton said he could tell within 15 seconds that the woman was dead.

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"She was really cold," he said.

"There was blood smeared on the wall. So either she walked there and fell over and touched the walls, or somebody else had touched her."

A worker from the office building opposite the apartment said an email went round their floor advising that the blinds had been shut and not to open them.

Without the blinds office workers would have had a clear view of the tense scenes unfolding on the awning.

James Chen lives in a building near where the woman's body was found.

He had spoken to a neighbour of the man who didn't hear anything unusual this morning.

A resident of the building told News.com.au no one could access level 23 or higher.

One woman told news.com.au police had stopped her leaving the building.

She said her young children were alarmed to hear about the death of the young woman.

After the passer-by contacted emergency services early on Monday morning, police and paramedics arrived on the scene.

WHERE TO GET HELP:

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