Dozens of beaches in southern Queensland, popular with tourists, were declared a disaster zone yesterday, with 40 miles of once-pristine sands fouled by an oil slick and local wildlife under threat.

Up to 100 tonnes of oil are believed to have spilled from a cargo ship, Pacific Adventurer, after its hull was punctured in seas whipped up by a tropical cyclone. Thirty-one containers of ammonium nitrate fertiliser were knocked off the ship’s deck during the storm, and are thought to be lying on the ocean floor.

Worst affected are beaches on Bribie and Moreton Islands, just north of the state capital, Brisbane. Both are national parks, and home to a range of sea birds and marine life, including turtles, dolphins and pelicans. A handful of oil-soaked creatures have been caught and cleaned, but wildlife officials fear many more are likely to be affected.

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Oil has also come ashore on the Sunshine Coast, on the mainland. The Queensland premier, Anna Bligh, said: "It may well be the worst environmental disaster Queensland has ever seen." She warned that the government would be presenting the owners of the Hong Kong-flagged ship with a "very large compensation claim".

Officials said the situation was likely to worsen, with sludge expected to wash ashore for weeks. They are also concerned about the impact of the fertiliser, that could cause major algae blooms, choking marine life in Moreton Bay.

Scores of environmental clean-up workers spent yesterday raking up sticky black sand and trying to prevent oil from spreading into nearby mangrove swamps and waterways. The declaration of a disaster zone gave authorities the power to close off public access to stricken beaches.

The Australian Maritime Safety Authority said the ship, owned by the British company Swire Shipping, had been detained after being brought to port, still leaking oil. The state government accused Swire of initially misleading it about the size of the spill and its likely impact. The company faces fines of up to A$2m (£941,000) and could be liable for up to $250m more in penalties for causing environmental damage. In a statement, Swire said it "regrets the extent of the environmental pollution caused by spills of heavy fuel oil from the ship and the company is offering assistance with the clean-up". The company and its insurers were in discussion with the government about clean-up costs, it said.

Pacific Adventurer was on its way from the port city of Newcastle, in New South Wales, to Indonesia when it was caught in last throes of Cyclone Hamish. As the ship was tossed around, the containers – containing 620 tonnes of ammonium nitrate – fell off the deck and ruptured a fuel tank.

The clean-up operation, expected to cost millions of dollars, is a delicate one, as stormy seas and high tides are continuing to erode beaches, carrying the oil into nearby rivers. However, large waves are also helping to break up the slick, according to the Environmental Protection Agency.

A spokesman for the agency said: "The flow-on effects of oil spills can be substantive. At the moment, we’re very lucky … but obviously, we’re worried it might escalate."