Detectives believed the person, who was yet to be formally identified, had been dead for about two weeks.

Police were contacted on Sunday morning by a member of the public about an abandoned green Honda Accord sedan on East Street in Lidcombe, near Rookwood Cemetery.

"They showed up, had a look, and there was a body in there," a police spokeswoman said.

Homicide squad detectives were quickly called to the scene and a crime scene was established. Forensic investigators were also called.

"The car has been abandoned there for the last two to three weeks," said neighbour Justin Salar, 22. "We're all pretty shocked at the moment."

Police said the death was being treated as suspicious.

Late on Sunday, sources said it was believed the deceased was male, and a form of identification had been found in the car.

As forensic officers combed the car and surrounding area for clues, detectives were seen peering into the boot of the car and taking notes.

East Street remained closed and two bus routes were diverted as inquiries continued.

Neighbours said it was not uncommon for students from nearby TAFE and university campuses to dump unwanted cars in the area, and they thought this vehicle might have been one of those.

Investigators appealed for anyone who might have any information or witnessed any suspicious behaviour near the car to contact Crime Stoppers.

– with Lisa Davies