NEW Zealand said it was preparing for an environmental disaster today over fears a container ship stranded off the North Island would break up and spill oil into the pristine Bay of Plenty.

The 47,000 tonne container vessel "Rena", which hit a reef off the North Island earlier this week, has already created an oil slick more than 5km long that has killed a number of seabirds.

But the pollution would increase immensely if the ship broke up on the Astrolabe Reef, releasing the 1700 tonnes of heavy fuel oil on board into the sea.

Environment Minister Nick Smith told Fairfax Media the accident "has the potential to be New Zealand's most significant maritime pollution disaster in decades".

Transport Minister Steven Joyce said salvage teams were scrambling to remove oil from the stricken vessel to protect the Bay of Plenty, one of the country's top tourist spots and home to whales, dolphins, seals and penguins.

"The difficulty is that the situation is deteriorating and according to the advice I've received, there's the possibility it could break up and sink," he told the New Zealand Herald.

"It's certainly serious, what's going on there. They're certainly moving as fast as they can. It's been a bit frustrating for everybody in terms of getting the right equipment to achieve the removal of the oil and containers."

Maritime New Zealand said it was preparing for the existing oil slick to hit the coast after dispersants sprayed into it from aircraft proved ineffective.

The agency found four dead birds yesterday and dispatched wildlife rescue teams today to scour Bay of Plenty beaches for oil-covered animals and birds.

It is not known why the ship ran aground in the early hours of Wednesday morning. None of the 25-man crew was injured in the accident.

AFP