Fresh oil leaked from a container ship stuck on a New Zealand reef Tuesday, as bad weather halted both salvage work on the vessel and a massive pollution clean up on the coast.

Maritime New Zealand (MNZ) said the Rena was pounded by four metre (13 foot) swells and 35 knot (65 kilometres per hour) winds overnight, forcing salvage crews pumping oil from its fuel tanks to abandon their work.

It said about 90 tonnes of oil had been offloaded, leaving more than 1,200 tonnes still on board the vessel, which sparked New Zealand's biggest sea pollution disaster when it ploughed into a reef on October 5.

Some 300 tonnes have already fouled beaches on the North Island's Bay of Plenty, killing at least 1,300 birds in the environmentally sensitive area, with the final toll expected to be significantly higher.

MNZ salvage chief Andrew Berry said more oil had spilled from the Liberian-flagged ship but the amount appeared to be small and prevailing winds were driving it offshore.

Huge cracks have opened on the vessel's hull since it grounded, threatening to shear it in two, and Berry said the stop-start salvage operation would take a long time.

"Every drop of oil that we can get off the Rena is one less drop that potentially can end up in the environment," he said.

"But given the huge range of variables that can affect operations, it's going to be a long, slow process."

Poor conditions expected to continue until Wednesday also forced MNZ to stand down an army of more than 5,000 volunteers involved in cleaning up blackened beaches.

"This is a last minute change due to bad weather," it said. "Our thanks and apologies to those volunteers who were ready for action."

MNZ has described the salvage operation on the Rena, which is cracked and listing badly on the Astrolabe Reef 22 kilometres (14 miles) offshore, as one of the most complex it has ever faced.

The viscous oil has a treacle-like consistency and has to be pumped through an eight-centimetre (three-inch) pipe, with the crew ready to evacuate at a moment's notice if the ship begins to break up.

The Rena's owner, Greek company Costamare, has apologised for the disaster, while its charterer, the world's second-largest container shipping firm Mediterranean Shipping Company, has agreed to pay some of the clean up bill.

New Zealand has launched two investigations into how the ship hit the reef in calm conditions but its captain and second office have already been charged with operating a vessel in a manner causing unnecessary danger or risk.

Compared with some of the world's worst oil spills, the disaster remains small -- the Exxon Valdez which ran aground in 1989 in Alaska dumped 37,000 tonnes of oil into Prince William Sound.

But it is significant because of the once-pristine nature of New Zealand's Bay of Plenty, which contains marine reserves and wetlands and teems with wildlife including whales, dolphins, penguins, seals and rare sea birds.