SYDNEY, March 17, 2009 (AFP) - Australian authorities on Tuesday launched a major marketing blitz to attract back tourists after a toxic spill off the popular northeast coast blackened beaches and threatened wildlife.

Visitor numbers have plunged since the Hong Kong-flagged Pacific Adventurer spilled a combination of oil and ammonium nitrate fertiliser into waters off Queensland's pristine northeast.

"This campaign will very clearly say we're back in business," said Paul Lucas, Queensland state's deputy premier.

Queensland's government said it would commit AUD$750,000 dollars (495,000 US) to an intense marketing campaign urging tourists to return.

The spill - comprising 250,000 litres of oil and 600 tonnes of chemical - and subsequent rush of tourist cancellations had put 13 travel operators out of business, said the tourism authority's Anthony Hayes.

"It's been absolute hardship for them over the past week," said Hayes.

But only five percent of the state's beaches had been affected and much of the worst-hit Moreton Island and Sunshine Coast areas were open again, said Lucas.

A navy minesweeper will on Thursday arrive to help find 31 containers of the ammonium nitrate, lost overboard during cyclonic weather last week, while the captain and crew remain on board pending the outcome of investigations.

Queensland authorities have accused the ship's operators of misleading them about the extent of the disaster, initially estimating it had lost 20-30,000 litres of oil but then revising the figure tenfold.

The ship's owner, Swire Shipping, has "categorically" denied lying. It has said the captain and crew will remain on the vessel as long as required by authorities and will fully cooperate with investigations.

The company faces the prospect of heavy fines and a multi-million-dollar clean-up bill.