Salvage workers in New Zealand have launched an attempt to remove about 1,700 tonnes of oil from a container ship that ran aground off the coast last week.

The Liberian-flagged Rena, with 2,100 shipping containers on board, struck the Astrolabe Reef about 12 nautical miles from Tauranga Harbour early Wednesday and is listing at about 10 degrees.

Birds and penguins have been sighted covered in oil and there are fears the ship could break up in gale-force winds, predicted to hit Monday.

The vessel, operated by the Mediterranean Shipping Co., was headed for the Port of Tauranga to load cargo when it became stranded.

A helicopter sprays a dispersant on an oil slick from the container ship MV Rena, which is stuck on a reef off the coast of Tauranga, New Zealand. Alan Gibson/New Zealand Herald/Associated Press

Maritime New Zealand, the agency responsible for shipping in the region, says about 20 to 30 tonnes of oil has spilled into the Bay of Plenty and has formed a five-kilometre-long slick.

The New Zealand navy has sent four ships to help break up the oil. Planes and helicopters have been spraying chemicals on the slick in a bid to break it up, but so far, that has failed.

The agency said in an update late Sunday that a barge, the Awanuia, had pulled up alongside the Rena and was to begin pumping fuel from the stricken ship. The operation is expected to last at least two days, although it could be delayed by bad weather.

Breck Gartrill, head of the New Zealand Wildlife Health Centre, said the oil is killing penguins and seabirds.

"From tip to toe they are covered in black sticky gunk, matting up all their feathers, right down to the skin. They've ingested it and they are starting to get anemic, which is part of the toxic effect of the oil," he said.

A rehabilitation centre has been set up in Tauranga for animals that came in contact with oil.

With files from The Associated Press