New Zealand on Tuesday declared its worst maritime pollution disaster, as oil gushed into a pristine bay from a stranded container ship being pounded in heavy seas.

The crippled container vessel Rena, which hit a reef off the North Island coast last Wednesday, leaked up to 300 tonnes of heavy fuel after being further damaged in the storm, dwarfing an initial spill, authorities said.

"I'd like to acknowledge this event has come to a stage where it is New Zealand's most significant maritime environmental disaster," Environment Minister Nick Smith told reporters at Tauranga.

Smith said there was little authorities could have done to prevent the disaster in the environmentally sensitive Bay of Plenty, where beaches have already been fouled with oil and some wildlife found dead or contaminated.

"It is my view that the tragic events we are seeing unfolding were absolutely inevitable from the point that the Rena ran onto the reef in the early hours of Wednesday morning," he said.

The latest spill was much larger than an initial leak of 20 tonnes after the Liberian-flagged vessel ploughed into the reef, 22 kilometres (13.6 miles) offshore.

Authorities have warned coastal residents to stay away from the viscous sludge, describing it as toxic, but many have ignored the advice and formed their own clean-up teams.

Earlier Tuesday, the stricken ship issued a mayday and nearby boats, including six navy vessels, scrambled to evacuate a salvage crew when the Rena shifted position on the reef as it was pounded by huge waves.

One sailor received minor injuries when a Rena salvage crew member fell on him while transferring to a navy vessel.

Maritime New Zealand (MNZ) said one of the Rena's four fuel tanks had ruptured but was unable to say whether it was in the stern, where most of the oil is stored, or the largely empty tanks in the front, which has sustained the most damage.

The authority said the Rena's list had worsened to about 15 degrees to starboard as it was hammered by swells of four to five metres. It estimated the latest spill at between 200 and 300 tonnes.

Officials have warned that New Zealand faces a major disaster if the Rena breaks up on the reef and releases all 1,700 tonnes of oil on board, describing fuel offloading as the "top priority".

MNZ director Catherine Taylor said the vessel appeared stable but "the weather forecast is not good" and that further shoreline pollution was inevitable.

The vessel sustained further damage to the front of its hull in the rough seas and additional flooding in its forward holds, MNZ said, but added this might help to stabilise the ship, preventing it moving around on the reef.

"They're not thinking that the vessel is going to break up at this time. They're aware it's resettling into a new equilibrium," MNZ salvage unit manager Bruce Anderson told reporters.

The drama at the accident site came as clean-up efforts were underway on Bay of Plenty beaches where blobs of tar-like oil that locals said resembled "black jellyfish" began to wash up on Monday.

Compared to some of the world's worst oil spills, the Rena disaster remains small -- the Exxon Valdez running aground in 1989 in Alaska dumped 37,000 tonnes of oil into Prince William Sound.

But it is significant due to the pristine nature of New Zealand's Bay of Plenty, which teems with wildlife including whales, dolphins, penguins, seals and rare sea birds.

Authorities said a considerable amount of oil was expected to come onshore in coming days, with the spill already killing some sea birds and several Little Blue penguins requiring treatment after being coated in the muck.

Local residents said they had collected large numbers of dead birds and fish on beaches, while a wildlife rescue centre said it expected to reach its capacity of 500 birds in coming days.

"The next 24-48 hours are pivotal if an environmental catastrophe is to be averted," World Wildlife Fund NZ marine programme manager Rebecca Bird said.

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