An ancient Aboriginal rock art site, believed to be tens of thousands of years old, has been discovered in the suburbs of Sydney.

The site, located in Sydney's north shore area, is in a pocket of pristine bush that has kept the art hidden for generations.

Locals were not aware of the presence of the site because it was either obscured behind vegetation or dismissed as graffiti.

The site was discovered by chance, when Sydney Water investigated a traditional fishing hook found in the soil.

Sydney Water has refused to reveal its location for fear it could be vandalised.

The discovery of a traditional fishing hook in the area led to the ancient Aboriginal rock art site. ( ABC: Anne Barker )

"It was found on the top of the midden site, and quite exposed," said Yvonne Kaiser-Glass, a heritage officer at Sydney Water.

"We wandered down here and found this. We'd really gone to see the water pool."

While the art - mainly hand stencils - was yet to be scientifically dated, it had been photographed and colour-enhanced in Photoshop to make the natural ochres more visible, and to differentiate it from newer art or graffiti.

Rock art may have been mistaken for graffiti

While there was evidence people had been coming past the site for decades, nobody appeared to have recognised the art's true worth, possibly mistaking it for graffiti.

Traditional owners are only now learning of the existence of the artwork left by their ancestors, who for thousands of years camped beside a nearby waterhole, eating eels and fish and sheltering from the weather.

This photo of the Aboriginal rock art site has been enhanced in Photoshop to bring out the original ochre colours. ( Supplied: Sydney Water )

The hand stencils depicted life as it was in ancient times, Col Davison from the Metropolitan Local Aboriginal Land Council in Sydney said, including such everyday images as eels, a spearhead and a crescent-shaped moon.

"That's an eel on the roof," said Mr Davidson, pointing to some drawings under a rock ledge.

"These are hand stencils, and judging from the size of these, they would have been women and children. So you could imagine they'd be here, resting."