Oil from a stranded container ship began washing up on New Zealand's Bay of Plenty on Monday, as bad weather hampered clean-up efforts and attempts to salvage the crippled vessel.

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

By late Monday, a six kilometre (3.7 mile) stretch of coastline was contaminated with a tar-like toxic ooze which smelled strongly of petrol, and the pollution has also reached Matakan Island, a sea bird nesting site.

MNZ said it was inevitable that the slick, formed when an estimated 20 tonnes of oil leaked from the stricken vessel, still stuck fast on a reef 22 kilometres offshore, would contaminate more beaches in coming days.

Officials closed affected beaches and warned people to avoid the oil, saying clean-up teams would swing into action at high tide on Tuesday, when the strong winds driving the slick onshore were expected to ease for a period.

"If we rush to clean all the oil off the beach now we will just be back there in a few hours to do it again, which isnt the best use of our resources," it said.

But locals, who have criticised the speed of the oil slick response, began their own clean up, shovelling the viscous sludge into plastic bags then dumping them above the tidal zone on a coastline littered with dead fish.

Matthew Morrow, a keen surfer who has lived in the area for 10 years, said he had joined about 40 people and felt devastated when he saw large blobs of oil that "looked like black jellyfish" marring the beach.

"It's just sad," he told AFP. "It's disgusting, tragic...it seems to be up to the public to deal with it."

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 prepared for the worst as stormy weather 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 36-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.

The agency said late Monday that there were "no obvious signs of deformation of the vessel", which was battered by heavy seas and wind gusts the official metservice reached 90km/h (56mph).

The salvage team had hoped to remove the oil before the bad weather arrived but MNZ said safety concerns meant that the operation had to be suspended after about 10 tonnes had been pumped onto a tanker moored beside the ship.

It said it needed at least two days of uninterrupted pumping to clear the oil from the vessel. The metservice predicts strong winds will continue on-and-off until Friday.

Further complicating the operation, authorities said four shipping containers on the Rena were full of the hazardous alloy ferrosilicon.

Some 250 people, including specialists from Australia, Britain, Holland and Singapore, have joined the oil slick response team, with 300 defence personnel on standby and expected to help the shoreline clean-up.

Transport Minister Steven Joyce said oil could wash up on the coast for weeks to come.

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.