A woman on my rock art tour who works in tourism reels off the usual rushed itinerary for overseas visitors to Sydney: "They put them in a bus, drive them to Bondi and Manly, have a day in the Blue Mountains on the Scenic Railway then it's back in the bus and that's Sydney."

It's all a bit wham, bam thank you ma'am, with scant attention paid to Sydney's rich indigenous heritage.

Paul Pickering feels the same way. "Where are all the tourists," he asks, as we walk down an empty track in the Ku-ring-gai Chase National Park, home to about 1,500 pieces of Aboriginal rock art. It's the largest concentration of Aboriginal sites in Australia, eclipsing Kakadu National Park, in the Northern Territory, which has about 5,000 sites but over a much greater land mass, says Pickering.

After leaving his job in the finance industry last year, Pickering started Sydney Out Back, which runs rock art tours in the national park that borders Sydney's northern suburbs and along the Hawkesbury River. He's passionate about the national park and believes it's underappreciated.

"I've met some people who live in Curl Curl [in the nearby Northern Beaches region] who haven't even been here," he says.

Sydney is spoilt for national parks. Within an hour of the city limits sits the Royal National Park to the south and the Blue Mountains to the west, and 45 minutes from the city lies the 14,977 hectare Ku-ring-gai Chase National Park.

"What we are seeing today is a time capsule from pre-European times," says Pickering, near the park entrance. A few metres further on is a picturesque clearing that reveals a sandstone rockface, thick bushland and a creek where water - overnight rain - is flowing fast down rocks. It's one of those brilliant, sunny winter days, when the smell of the eucalypt is especially strong and fresh. The city feels very far away.

Until recent times, the area remained relatively unexplored. It wasn't until the 1960s that roads were built into the park, with that to Cottage Point (where there is a lovely restaurant) not opened until 1967.

We are visiting on a Wednesday and come across only five other people - all at local beauty spot West Head lookout, which has a great view across to Palm Beach (the set of television soap opera Home and Away), Barrenjoey lighthouse and Lion Island. The beach beneath the lookout is the site of the first recorded contact between New South Wales governor Arthur Phillip and indigenous Australians, in 1788.

Our Aboriginal guide, Les McLeod, passes around old logs from the NSW National Parks and Wildlife Service, cataloguing all the known rock art in the region: what the site is, what it depicts and its level of degradation. The area's significant Aboriginal sites include rock engravings, art sites, burial sites, caves or shelters (some with shell deposits), marriage areas, men's areas, women's areas, birthing areas, midden sites, stone arrangement sites and tool manufacturing locations.

We are on Guringai land, and McLeod has the elders' permission to hold tours and explain the rock art. He smears ochre on our faces and hands and we stand in a circle as he claps message sticks.

The first rock art he takes us to is carved into the ground on a flat, treeless ledge in the park. The engravings here are outlines created on rock surfaces by pecking, hammering or scraping. Their exact age isn't known but Pickering estimates they're between 800 and 1,500 years old.

We have had to bush-bash to get in here, and a small sign is the only hint of what lies off the track. McLeod squirts water over the rocks and an enormous engraving - metres long - of a serpent appears.

"A lot of Aboriginal people believe they were created from animals - there are engravings here of wallabies, fish and emus," says McLeod. "Sydney sandstone is easy to engrave but easy to fade. The Guringai people would have visited a couple of times a year to re-engrave it."

He and Pickering speculate that some of the patterns on the rocks resemble the Milky Way and may have been intended as an astrological guide. The site would have also been a meeting place for ceremonies and corroborees (at which Aborigines interact with the Dreamtime through dance, music and costume), says McLeod.

Then it's on to Akuna Bay. From the very posh marina, we sail up the Hawkesbury River on a charter boat in order to see rock art that's visible only from the water.

"Sometimes the best way to see Aboriginal sites is actually in the natural environment, as opposed to in a museum or art gallery," says McLeod, as the boat draws close to a small cave on the water's edge where there are ochre hand stencils from a group of Guringai men (and, judging by a smaller handprint, a child). The stencils would have been a way of letting fellow clan members know that this particular cave or ledge was a safe place to stay, McLeod explains.

Further up the river is a rock paint-ing of a fish just above the water line. This would have been a signal to others that fish could be found at this spot, says McLeod.

The Hawkesbury is a magnificent river, with bush land running all the way down to the water, and has a particular quality of silence broken only by the occasional lyrebird or kookaburra. The part we're in is fairly remote (and ours is the only boat in these waters), but McLeod doesn't want us going much further. There are significant Aboriginal sites upriver but they have been vandalised with racist graffiti.

It's a jolt back to reality after an afternoon exploring such a pristine environment. Nature may wear away at the rock art, but it's humans who pose the biggest threat.

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