Our host on the 18-seater turbo-prop plane from Brisbane to Hervey Bay was Cameron, who had ginger sideburns and tripled as flight attendant, baggage handler and pilot.

Our host on the 18-seater turbo-prop plane from Brisbane to Hervey Bay was Cameron, who had ginger sideburns and tripled as flight attendant, baggage handler and pilot. "The weather's not good up there," he said, vanishing into the cockpit after reciting the safety drill at breakneck speed. After a bumpy 55 minutes, we skidded on to the wet runway at Hervey Bay airport, a tin shed located a short walk from the wharf. There, a catamaran, full of Belgians writing postcards in yellow waterproofs, was waiting to whisk us across the narrow stretch of water to Fraser Island.

Even in the driving rain, Fraser is magnificent. This is the world's largest sand island, and a must-see destination for nature lovers who tire of the crowds of Australia's Gold Coast. It is World Heritage-listed, thanks to its spectacular ancient dunes and other natural formations. It is a bushwalker's dream, a fisherman's paradise, a Mecca for four-wheel-drive maniacs who hare across the long, empty beaches. It sparkles with exquisite freshwater lakes with white, sandy bottoms and water that is incredibly pure and clear. This is a pristine environment full of shimmering creeks and rainforests that sprout - almost miraculously - out of the sandy earth. At Lake McKenzie, the sand is so fine that you can use it to exfoliate your skin, or even clean jewellery.

To get around this wilderness, a four-wheel-drive is a necessity, though you can take tours tailored to your tastes. Driving is an adventure in itself; you feel like you are competing in the Paris-Dakar rally as you bounce and rock your way along the narrow, rutted tracks. It can be perilous too - at Seventy-Five-Mile Beach we saw a Range Rover sinking serenely into the shallows, resisting attempts to haul it out of its soft, sandy prison. Yidney Jack, who owns the island's sole tow-truck business, is a man permanently wreathed in smiles.

It is not only visitors who get bogged down, though. How we laughed when Graham, our guide, told us that tour groups sometimes have to get out and push. Two days later, that is exactly what we were doing, all 25 of us, heaving with all our might as Graham ground the gears and rolled back and forth in a futile effort to get the four-wheel-drive bus moving.

Stranded in a lonely spot, we walked 7km to Indian Head, with the rain bucketing down, lightning flashing and thunder cracking overhead. Indian Head had its compensations - wonderful views, and an enormous glacier-like collection of dunes stretching out beneath us. From this high point, you can gaze out over the churning ocean and spot sharks, dolphins and - from July to October - humpbacked whales.

Driving is fun on Fraser, but walking is even better. One of the best routes is from Central Station, once the headquarters of the forestry industry, along Wanggoolba Creek to Pile Valley, where timber was exported to make marine piles for the Suez Canal and Tilbury Docks. Wanggoolba, sandy-bottomed and fringed by rainforest, flows through a shady gully and is known as Silent Creek because it does not babble. The water - so clear that, if you step back, you cannot see it - has no rocks or vegetation to dash against. It comes straight out of the abundant underground water table.

Fraser was logged for more than a century, and was also sandmined; both activities ended after long campaigns by conservation groups. Aborigines lived in the area (part of the traditional country of the Butchulla people) for at least 5,000 years, but were wiped out by European weapons and diseases. Hostility was fuelled by the lurid stories of Eliza Fraser, who was shipwrecked on the island in 1836 and claimed that her husband was speared and eaten by the natives. In reality, he died of natural causes, but Fraser is still named after her.

There are plenty of short walks and, for the dedicated hiker, a newly opened "Great Walk" - a 90km trek that takes in most of the island's natural attractions, including the beautiful lakes that perch among sand dunes. Basin Lake is a tranquil oval, ringed by vegetation, where tiny turtles flipper through the shallows, raising their heads like periscopes to gulp air. The irrepressibly cheerful Graham had told us about Fraser's seven species of poisonous snake, including the death adder, which rears up and snatches birds in flight. But at Basin Lake, the worst hazard was summed up by a sign that warned: "Caution: falling pine cones".

Lake Wabby, another great destination, is reached by walking 2km across a sand dune so broad that you could be in the middle of the Sahara. A sheer slope descends to the lake; rolling or running down it into the cool, limpid water is a popular pursuit. Shoals of catfish and jungle perch glide past, just below the surface.

At Lake McKenzie, surrounded by the whitest of dazzling white sands, you can see three distinct shades of blue as the water gets deeper. The water is so warm and fresh that it tastes faintly sweet and is almost fragrant. The dunes are backed by tall paperback trees that incline at a worshipful angle.

You may be joined on your towel by a goanna, a type of large lizard, pausing on its leisurely stroll across the beach. Dingoes also hang around McKenzie, stealing cameras and watches. Fraser has 150 of Australia's purest-bred dingoes, which roam in packs and can be dangerous. The main campgrounds are now enclosed by dingo-proof fences, and signs everywhere explain how to be "dingo-smart".

Along Seventy-Five-Mile Beach is a series of "sights", including the Maheno Wreck, the silhouetted remains of a rusting ship. At Eli Creek, you can lie on your back and let the fast-flowing current carry you downstream. Here you will also find the Pinnacles, a stunning formation of rainbow-coloured sands.

On this island, which is the size of Surrey, there is a range of accommodation - numerous camp sites, and four or five other centres. The best is found at Kingfisher Bay, a resort that prides itself on its eco-friendly credentials (a worm farm turns sewage and kitchen waste into compost, would you believe?). The buildings are all of local timber, and enclosed by native vegetation. You can hire a "personalised ranger" - perhaps the ultimate luxury on an island whose charms, on each visit, never seem quite exhausted.

Sunshine Express ( www.sunshineexpress.com.au) flies between Brisbane and Hervey Bay, with fares from $103 (£40) one-way. A double room at Kingfisher Bay Resort ( www.kingfisherbay.com) costs from $165 (£65) a night. Fraser Island Explorer Tours and The Fraser Island Company both do day tours of Fraser from Hervey Bay, as well as extended tours. For more information visit www.frasercoast.org