New Zealand warned more oil was set to spill from a crippled container ship Monday, as looming bad weather threatened to halt the draining of fuel from the stricken vessel's tanks.

Salvage crews pumped 20 tonnes of fuel overnight from the Rena but about 1,300 tonnes remain on the wreck, which is listing badly on an offshore reef, and officials said removing it was a painstaking, dangerous operation.

Maritime New Zealand (MNZ) said more coastal communities in the environmentally fragile Bay of Plenty were braced for the slick created by 300 tonnes of oil that have already leaked from the Rena to hit their shores.

As an army of volunteers continued to collect black sludge from affected shores on the North Island bay, which teems with wildlife, the company that chartered the ship denied liability for New Zealand's worst sea pollution disaster.

But Mediterranean Shipping Company, the world's second-largest container shipping firm, said it was willing to shoulder an unspecified part of the rising clean-up bill, estimated at NZ$4 million ($3.2 million) so far.

"We are not liable in this situation but we are more than willing to assist and help wherever possible," MSC's Australasian managing director Kevin Clarke told reporters after a meeting with Environment Minister Steve Joyce.

Joyce said MSC had an obligation "as a responsible corporate citizen" to make a contribution as the Liberian-flagged vessel was under contract to it when it hit the reef.

With the salvage operation making slow progress due to the wreck's precarious position and intermittent bad weather, Joyce warned: "I would expect further spills to occur at different points of this exercise, so we've got a way to go yet."

MNZ salvage manager Bruce Anderson said a three-man crew worked overnight to pump oil from the vessel, which has has huge cracks in its hull and could break apart at any time.

"It was hairy," he told reporters. "This thing is groaning and creaking and making huge noises. It's a vessel dying."

Anderson said the salvage team was increased to nine on Monday, with workers scrambling to install more efficient pumps before a forecast deterioration in the weather Monday night.

But he said the sticky, viscous oil had to be heated before it could be pumped through an eight-centimetre (three-inch) pipe and the crew had to be ready to evacuate at a moment's notice,

He too expected further oil spills as the salvage operation dragged on.

"How much oil we don't know yet, when, we don't know yet," he said, adding that pumping was likely to halt Monday night if seas became choppy as expected.

Large cracks have opened in the Rena's hull about 90 metres (300 feet) from its prow. The front end is wedged on the rocky Astrolabe Reef, about 22 kilometres offshore, while the rear is floating in the sea.

The spilled oil has killed about 1,300 birds and fouled once pristine beaches, prompting 5,500 people to volunteer for shoreline clean up teams.