Salvage workers struggled Sunday to begin pumping oil from a stricken container ship off the New Zealand coast as approaching foul weather threatened to disrupt the recovery.

The spilled oil has killed about 1,000 birds and has been washed up on once pristine beaches.

Authorities began Sunday to reopen sections of popular beaches near where the cargo vessel Rena ran aground 11 days ago after a volunteer army removed more than 600 tonnes of oiled sandy waste.

The government is seeking a meeting with the shipping firm that chartered the Rena, saying it was not happy with their response to New Zealand's worst maritime pollution disaster.

Transport Minister Steven Joyce said he had called for talks on Monday with the Mediterranean Shipping Company, the world's second largest container shipping firm, saying "they have to step up and be part of this exercise".

The Rena remained teetering on the reef, but in calm waters, and Maritime New Zealand (MNZ) salvage head Bruce Anderson said approval had been given to extract the fuel overnight now an evacuation plan was in place.

Anderson said the ship was unstable and it was essential to ensure the salvage workers could be taken off if necessary.

About 90 metres of the Rena in front of a large crack in its hull is wedged on the Astrolabe Reef, but to the rear of the crack another 60 metres of the ship is hanging off into deep ocean.

Although the forecast was good in the immediate future, rising seas were forecast for Monday night.

It is believed 1,346 tonnes of oil remain on board the vessel, which was carrying 1,673 tonnes when it ploughed into the Astrolabe Reef early on October 5.

If the ship does not break up as the weather worsens, the oil will be pumped out at 20 tonnes an hour and the operation could take several days to complete.

"The speed of the operation will depend on a range of factors including weather, the stability of the vessel and the viscosity of the oil. This will be a long process," MNZ on-scene commander Nick Quinn said.

Without power from the ship's engines, salvage personnel have said that their main difficulty was heating the fuel, which has cooled to a dense consistency, making it harder to pump.

The mass volunteer effort to clear away hundreds of tonnes of sludge allowed authorities to reopen public access to a small section of beach on Sunday, although Quinn warned that changing tides and weather conditions could bring more oil ashore.

"People shouldn't panic if they do see some oil around there. It's been three days since we got any fresh oil off the ship and so the oil that is out there is weathered and less toxic than any fresh oil," he said.

"We are here for the duration of this response. When oil turns up on the shore, we will get it clean and open to the public as soon as we can. And if we need to, we will do that again and again."

The Rena was off course when it ran aground and the New Zealand government has accused the captain of attempting to take a short cut on his way to the port of Tauranga on the east coast of North Island.

The captain and the officer on navigational watch when the Liberian-flagged ship ran aground have been charged with operating a vessel in a manner causing unnecessary danger or risk.

The charge carries a maximum penalty of one year in jail.

While the salvage operation continued, the New Zealand navy was patrolling the area locating containers which had fallen from the teetering vessel and posed a hazard in the shipping channel.

Oil slick ship ran aground as captain 'cut corner'
Tauranga, New Zealand (AFP) Oct 15, 2011 - The vessel at the centre of New Zealand's worst maritime pollution disaster ran aground because the captain was taking a short cut, the New Zealand government alleged Saturday.

The accusation was made as salvage crews prepared to pump oil from the stricken cargo ship Rena, which ran aground last week.

Anger is mounting in New Zealand over the fuel leak, with popular beaches on the North Island's east coast coated in oil and off-limits to the public, and more than 1,000 dead and oil-soaked birds recovered.

There were indications Saturday the leak has been stemmed, but the ship's agent has said the six Filipino crewmembers who are still in New Zealand are being kept at an undisclosed location amid fears for their safety.

Environment Minister Nick Smith said it appeared the Rena hit a reef off the resort area of Tauranga when the vessel was trying to get to port quickly.

"I can't confirm that. But it appears from the charts that they were in a rush to get to port, went full bore, cut the corner, and hit the reef," Smith told TV3's The Nation programme.

The ship's captain and the officer on navigational watch when the ship ran aground have already been charged with operating a vessel in a manner causing unnecessary danger or risk.

The charge carries a maximum penalty of one year in jail.

Meanwhile, Maritime New Zealand (MNZ) salvage head Bruce Anderson said divers working around the ship reported it was not as stable as earlier believed although observation flights saw no fresh oil leaks on Saturday.

"Divers have indicated the stern is in a precarious situation as a portion of the vessel is still floating in deeper water, while the front of the vessel is firmly grounded on the reef," Anderson said.

Salvage crews working on board were expected to start pumping oil to a nearby tanker on Sunday.

It is believed there are still 1,346 tonnes of oil on board the Rena while about 330 tonnes have leaked into the ocean in an ecologically sensitive area teeming with wildlife, with 88 containers also falling into the water.

Matthew Watson from the salvage company Svitzer told Radio New Zealand a team on a fuel pumping barge half a nautical mile away had been testing equipment to remove the remaining oil.

Their main difficulty was finding a way to heat the fuel, which has cooled to a dense consistency and the ship's engines no longer have the power to warm it, he said.

On shore, nearly 1,000 dead birds have been recovered and a wildlife facility is caring for 110 injured birds as a volunteer army of 2,000 people were on their hands and knees meticulously cleaning beaches.

"You can't get machinery in here, for starters. So you've got to get people down here on their hands and knees to pick it up. Otherwise no one else is going to do it," said Wayne Sayers on Papamoa beach.

Team leader David Eddy added: "Initially it was just on the surface, but now we're going a lot deeper. A couple inches deeper. We've found some large clumps of it."

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 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.