It is an untouched tropical paradise, right on the edge of one of the seven wonders of the natural world: a place so beautiful it makes a balmy Caribbean isle look positively ordinary. And it can all be yours for just £17m — less than the price of an average Barbados plantation mansion. There is one tiny downside, but let’s come to that in a moment.

On face value at least, Keswick Island in Australia’s Whitsunday Passage lives up to its selling agent’s hyperbole.

Off the north Queensland coast, it’s right on the edge of the Great Barrier Reef, and is in pristine condition. A huge chunk — some 80% — of its 1,235 acres is given over to national park, and there are just 15 houses and one guesthouse. All services are established, there’s an airstrip, a detailed tourism and residential masterplan and, crucially, the necessary permits for an admittedly tightly controlled but eco-friendly, mixed-use development. And now the whole island — or rather, its 93-year lease — is up for sale.

Sydney businessmen Ralph McCann and Vince Alexander bought it in 2002, planning to carry out a mixed-use development, which has finally been approved, after years of wrangling, by a complex array of Australian authorities. It would see the creation of a rigidly eco-friendly island town of 3,000 residents and 850 dwellings — houses, flats and duplexes — in five residential clusters; there would be a five-star hotel and other tourist developments, two 100-berth marinas and a school.

The planning permission has taken years to win, and foreign-investment rules mean would-be developers must have an Australian partner on board. Cars are banned, and each dwelling is subject to a detailed architecture and amenity code, plus approval by committee. Seventy percent of energy used must come from renewable resources. But just as they’ve won the green light, and with massive yields on the horizon, McCann and Alexander are selling up: they realise they’ve bitten off more than they can chew.

“We’re not really developers,” says McCann. “We can’t do justice to it. We know our limitations — you will be building and maintaining a town in the ocean.”

So that’s where things become interesting. Because, according to Alex Caraco of estate agency Coldwell Banker Australia, news that Keswick Island’s lease is up for sale has prompted strong global interest from both developers and individuals.

Caraco confirms there is serious interest from at least one British-based investor. He won’t name him, but confirms the man and his father have been behind a string of US West Coast developments, and that lawyers have been sent down under to do a detailed inspection. And yet another potential buyer, interested in keeping it as a private hideaway, has asked the agent to find out whether or not the island’s existing residents would be willing to move.

“There is nothing like this if you want to own your own island,” says Caraco. “The only comparable development (in Australia) that offers residential opportunities is on Hamilton Island, which is intensely developed with high-rise.”

Now for the catch. Scheduled flights from the UK will take 18 hours to Brisbane, the Queensland capital, then you’ll need to hop to the nearest coastal town, Mackay, and again to the island, which is a 10-minute flight off the coast. But then again, if you’re an eco-friendly multi-millionaire, perhaps with a young family, and hankering after your own pristine and protected getaway — think Gwyneth Paltrow and Chris Martin, or Zac and Sheherazade Goldsmith, for instance — what better spot to have, as you’ll probably already own the private jet that can get you there in comfort. At £17m, it’s a bargain compared to other private islands.

Thatch Cay, for instance, billed as the last truly “virgin” island in the US Virgin Islands, is also for sale through Coldwell Banker Morrison’s, but you’ll pay £13.8m for just 230 acres, and there’s no infrastructure at all. The average temperature in the tropical Whitsundays from December to February is 30C, making it the perfect winter-sun destination.

The Whitsundays are no stranger to celebrity residents. The former Beatle George Harrison was one of nearby Hamilton Island’s most famous residents — he bought the second house built there during its development 20 years ago. The clifftop mansion, complete with meditation nooks, is still held by Harrison’s estate.

Despite their prominence, the Whitsundays are relatively undeveloped — Keswick is expected to be the last to win approval — and fewer than a dozen of the 74 islands in the group are inhabited. British tourists make up the majority of international visitors to the area, and while overall tourist numbers are steady but not spectacular, optimism abounds: property prices there have trebled since 2000.

If your pockets aren’t deep enough to find £17m, of course, you could just buy a tiny slice of Keswick. Former Swissair pilot Phil Robinson bought his block there two years ago for £58,000. He spent £151,000 building a house, and now has it on the market for £320,000. “I was based in Zurich and was going to use it as a holiday house, but we decided to come home to live and now need to sell it,” he says, though life on the island is idyllic.

“You are right on the reef, you just wade out and you’re snorkelling on the reef in minutes.”

But you need patience if you are going to build on an island, he warns. There is a boom in the Whitsundays, and as a consequence, builders are hard to find. All materials need to be barged onto the island, so building can be a lengthy process. It is also twice as expensive to build on an island as it is on the mainland of Australia, but the end product is still reasonable by British standards when you convert into sterling.

And, as Phil Robinson says, “There’s nothing like sitting watching the sun go down over the reef from your own verandah.”

Keswick Island is for sale for £17m with Coldwell Banker Australia, 00 61 7 5538 8881, www.coldwellbanker.com.au; www.keswickisland.com.au

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