Police investigating after skeletal remains found in burnt-out cars in Royal National Park, Parramatta

"There's nothing to suggest they are linked," a NSW Police spokesman told ACM's Illawarra Mercury.

Shortly after midnight on Tuesday, a Toyota RAV4 was found alight on a fire trail at Waterfall. When the flames were extinguished firefighters found skeletal remains inside.

Two hours later in North Parramatta firefighters were called to a hatchback fully engulfed in fire. When the flames were extinguished a body was found inside the car.

Crime scenes have been established at both sites and the remains will be forensically examined to determine their identification.

On Tuesday morning, in a quiet corner of the Royal National Park at Waterfall rain was falling as police conducted line searches looking for clues.

They walked side-by-side looking through dense vegetation around the burnt-out car for evidence that may have been left behind.

The fire trail is located off McKell Avenue, around 1.5 kilometres south-east from Waterfall Public School and the nearest homes. Emergency services are yet to reveal how they were alerted to a burning car in dense bush so far from the nearest home or building.

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Volunteer firefighters from Waterfall and Helensburgh NSW Rural Fire Service brigades were the first to arrive at the blaze, and they were the ones to discover the skeletal remains inside the burnt-out car.

They then reported the incident to police who took over command of the scene.

These firefighters have since been offered assistance through the RFS' critical incident support service, Illawarra RFS acting district manager Cameron Wade said.

"There was a hot debrief at the scene," he said. "We have a critical incident team that exists and we have a debrief like we do for any job."

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

NSW Police Superintendent Barry Vincent said officers are following up on reports of gunshots heard near the North Parramatta fire around the time of the fire.