Oil from a stranded container ship began washing up on the shore of New Zealand's Bay of Plenty on Monday, as salvage crews battened down the crippled vessel in the face of a looming storm.

Maritime New Zealand (MNZ) said 'fist-sized patties' of oil from the container ship Rena, which hit a reef last Wednesday, were found on the beach at Mount Maunganui, one of the country's most popular tourist destinations.

Officials closed the beach and warned people to avoid the tar-like toxic ooze, which locals said smelled strongly of petrol.

The discharge is part of a five-kilometre (three-mile) slick blown ashore from the Rena accident site 22 kilometres off the coast.

MNZ said the slick, formed when an estimated 20 tonnes of oil leaked from the stricken vessel, was expected to pollute neighbouring beaches in the next 24 hours.

"We are expecting oil to wash up on the shoreline south of Mount Maunganui but we don't know how much," it said.

The spill has already killed a number of sea birds, with seven Little Blue penguins and two shags receiving treatment at wildlife rescue centres after being found covered in oil.

MNZ said it had also received unconfirmed reports of oil-coated seals in the huge bay at the top of the North Island, which is also home to whales and dolphins and has a shoreline dotted with sensitive wetlands.

Officials fear New Zealand will face its worst maritime pollution disaster in decades if the Rena breaks up and sinks, spewing a further 1,700 tonnes of heavy fuel oil into the sea.

Salvage workers were preparing for the worst as a storm, forecast to arrive late on Monday, bore down on the area.

They lashed down shipping containers on the vessel's deck and moved the fuel from damaged tanks at the front of the ship to more secure ones at the back, installing covers designed to seal it in if the Rena ends up on the sea bed.

MNZ said an evacuation plan was in place to airlift the 25-strong salvage crew from the ship if necessary, while sensors had been installed to monitor whether stress from rough weather was beginning to tear the hull apart.

"That's a possibility, the weather is something we're keeping a close eye on along with the ship's structure," MNZ salvage unit manager Bruce Anderson told Radio New Zealand.

"Obviously the potential for serious consequences is there, and we're under no illusions about that - that's why we're trying to work around the clock to get the oil off."

The official forecaster MetService issued a severe weather warning for the bay, predicting that heavy rain and winds gusting up to 90 km/h (56 mph) would hit late on Monday.

The salvage teams worked through the night, hoping to remove the oil before the bad weather arrived, but Anderson said that by Monday morning only 10 tonnes had been pumped onto a tanker moored beside the stricken vessel.

Transport Minister Steven Joyce said oil could be washing up on the coast for weeks.

He refused to comment on an NZ Maritime Union allegation that an MNZ inspection of the Liberian-flagged Rena before the accident revealed errors in its charts.

Joyce said two investigations were underway that would examine all aspects of the accident, including whether any of the crew had been drinking when the ship crashed.