A "strong or corrupted" batch of heroin is believed to be responsible for 13 drug deaths across Sydney in the space of a month, prompting an urgent warning from the New South Wales coroner.

Key points: 'Unusually strong' heroin being sold on Sydney streets

'Unusually strong' heroin being sold on Sydney streets 13 deaths attributed to bad batch

13 deaths attributed to bad batch Emergency doctor says use increasing over past 15 years

Coroner Michael Barnes said the deaths all occurred between May 2 and June 3, suggesting an "unusually strong or corrupted" supply of the drug was currently being sold on Sydney's streets.

He said in every case the deceased was found slumped on the floor with a syringe in their hand or needles and drug paraphernalia strewn next to their body.

Coroner Barnes said that indicated each person died shortly after injecting the substance.

"Street level heroin is rarely pure," he said. "[It] can sometimes be cut with drugs or compounds that can kill you in one dose.

"As state coroner, I deal with hundreds of deaths each year and it is part of my role to make recommendations that can save lives."

Mr Barnes said there was an urgency to bring the recent spate of drug-rated deaths to the attention of the public.

"The message is clear: if you're thinking of experimenting with illicit drugs — just don't do it," he said.

"Heroin has always been dangerous and these recent deaths highlight it's a gamble not worth taking."

Deaths spread across Sydney metropolitan area

The deaths all occurred within the Sydney metropolitan area, with three on the north shore, four in inner city suburbs, two in Penrith and two in Sydney's south-west.

The users were aged between 22 and 53, and two were female. The majority were middle aged men.

Commander of the NSW Police Drug Squad Michael Cook said there was no way for users to know exactly what they were taking.

"There is no quality control in the production process," he said.

"They will do whatever it takes to ensure a fatter profit margin can be achieved.

Frontline doctor reports steady rise in heroin use

A senior Sydney doctor said he had seen a recent surge in opiate-related drug deaths and overdoses coming through the emergency room.

Director of emergency medicine at St Vincent's Hospital, Dr Gordian Fulde, said heroin use had been steadily increasing since the "heroin drought" in Sydney 15 years ago.

"Before [the drought] we routinely, daily, saw heroin overdoses," he said.

"Heroin never went away ... but it isn't anything in proportion to what it used to be, but it's a slow increase."

He said many heroin users tended to be "lifestyle" users rather than recreational users and that could mean they were at greater risk of an overdose.

"It is a depressant, in other words it makes people sleepy, it makes their brain functions decrease," he said.

"If you keep taking stronger heroin ... it makes you stop breathing and it is the lack of breathing that kills you.

"You can die at the end of a needle because if you've got a very potent batch you basically stop breathing within minutes."