Gruesome remains found in two burnt-out cars in Sydney

After extinguishing the blaze, firefighters found a body in the driver's seat of the hatchback.

Superintendent Barry Vincent said police were treating the scene as suspicious and following up on reports of gunshots heard in the area about the time of the fire.

"There may have been a fuel source but what that is, I couldn't say definitively at the moment," he said on Tuesday.

The discovery was made just two hours after emergency services found a Toyota RAV4 ablaze at the Waterfall Flat car park at Waterfall in The Royal National Park south of Sydney.

Firefighters extinguished the blaze before finding skeletal human remains inside the rear of that vehicle.

"It is too early to tell whether those matters are linked in any way," Supt Vincent said.

Two crime scenes have been established and forensic examination is under way to determine the identities of the remains.

Homicide detectives are investigating the Parramatta blaze.

"Obviously we're keen to find out who this person is as soon as we can so we can make sure the necessary updates are provided to family who may be well unaware of what happened," he said.

Police are appealing for anyone with information or footage of the incidents to come forward.