Salvage crews readied Thursday for a badly listing container ship stuck on a reef to break up, deepening New Zealand's worst maritime pollution disaster as it enters a second week.

The stricken Rena's hull began to crack Wednesday after being battered by huge waves, threatening to sink the vessel and release a new tide of oil in the North Island's environmentally sensitive Bay of Plenty.

Maritime New Zealand (MNZ) said the vessel, which is teetering at a steep angle on the reef, was still in one piece early Thursday but three tugs were on standby in case it sheared apart.

"They've had three vessels out overnight monitoring the situation," MNZ pollution response manager Neil Rowath told TVNZ.

"They reported that at high tide the stern of the vessel was still floating and at the low tide it settled down slightly onto the reef, but she is still intact."

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

Transport Minister Steve Joyce said Wednesday that if the ship split, tugs 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 offload the oil, he said.

About 70 containers have toppled from the ship deck as it leans precariously on the the reef and they continued to wash up on the shore Thursday, with some landing on Mount Maunganui Beach, near the popular tourist spot of Tauranga.

Around 1,300 containers remain leashed to the vessel, including 11 filled with hazardous material.

With the heavy swells and strong winds that swept a black tide of oil slurry onto beaches around Tauranga easing, MNZ said a "massive" clean-up operation involving 500 people was under way.

"It's hard, dirty work, but with all the agencies involved and the community pulling together, we will get this oil cleaned up," MNZ on-scene commander Nick Quinn said.

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

The Rena's officer, who was in charge of navigational watch when the ship ploughed into the reef on October 5, was due to face court Thursday, 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.

His name was withheld over fears for his 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 to 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.