Salvage crews reboarded a stricken ship at the centre of New Zealand's worst sea pollution disaster on Thursday as authorities ordered people off oil-blackened beaches.

The crews, fearing that a breakup of the Rena was now inevitable, hoped to drain its fuel tanks to prevent further fuel from spewing into the environmentally sensitive Bay of Plenty before the ship shears in two.

Authorities ordered locals off a stretch of the coast after a hazardous substance entered the water from containers which fell from the ship. It is leaning precariously on a reef where it has been stuck fast since October 5.

Huge cracks opened up in the Rena's hull on Wednesday after a storm battered it against the reef, and while conditions eased Thursday, there was little hope it would remain in one piece.

"There's not very much room for optimism," salvage specialist Matthew Watson, whose company Svitzer is leading efforts to save the vessel, told Newstalk ZB.

"It looks as if a best-case scenario could be trying to at least get the oil off the vessel, then working out what to do with the vessel should she break up."

Asked if a split in the badly listing ship was inevitable, he replied: "You only have to look at that damage to the sides of the vessel."

Up to 300 tonnes of heavy fuel has already leaked from the Rena, fouling beaches and killing wildlife, with authorities fearing that all 1,700 tonnes on the vessel will be released if it breaks up and sinks.

Authorities said three tugs were on standby and if the Rena split they would try to hold the stern section, where most of the oil is stored, on the Astrolabe Reef, 22 kilometres (15 miles) offshore.

If that proved impossible, the tugs would try to tow the stern to shallow water, making it easier to eventually offload the oil.

Maritime New Zealand (MNZ) said a total of 88 containers had toppled from the ship deck, up from its initial estimate of 70.

It said one of the containers was filled with the hazardous alloy ferrosilicon, which can be dangerous when exposed to water and warned people to avoid containers which had washed ashore.

For the first time, MNZ ordered the public off the worst-affected 35 kilometre (22 mile) stretch of coast from Mount Maunganui to Maketu, clearing the way for a "massive" cleanup operation involving 500 people.

"It's hard, dirty work, but with all the agencies involved and the community pulling together, we will get this oil cleaned up," MNZ site commander Nick Quinn said as oil continued to come ashore in sluggish black waves.

Previously, the agency had only advised people not to go onto beaches in the area, one of New Zealand's most popular tourist destinations.

Residents living near affected beaches were advised to keep their windows closed as the toxic discharge released fumes that leave the eyes stinging and the throat raw.

MNZ said 200 dead birds had been recovered and it expected the toll on wildlife to increase substantially. It said that teams were trying to round up seals in the area and had caught five so far.

The Rena's second officer, who was on navigation watch when the ship ploughed into the reef on October 5, faced a court in Tauranga, charged with operating a vessel in a manner causing unnecessary danger or risk.

The ship's Filipino captain has already been bailed on the same charge, which carries a maximum penalty of NZ$10,000 ($7,800) or one year in jail.

Both officers had their names publicly suppressed over fears for their safety as anger grows in the community.

Investigations into the accident would examine whether the crew of the Liberian-flagged vessel were celebrating the captain's 44th birthday when the crash occurred, local reports said.

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, wetlands and teems with wildlife including whales, dolphins, penguins, seals and rare sea birds.