Fears grew on Wednesday that a ship stuck on a New Zealand reef would break up and release a new wave of pollution, as its two chief officers were charged over the nation's worst oil spill.

Prime Minister John Key said cracks had been found in the hull of the stricken container ship Rena, and the situation was precarious, with the boat teetering at a steep angle in stormy seas.

“We have identified stress fractures on the ship. We can't rule out the risk of the ship breaking up, that's certainly being monitored,” he told reporters near Tauranga, where already beaches have been fouled and wildlife found dead.

“I wish that boat wasn't spewing oil out into New Zealand's pristine sea, but it is and we're dealing with that.”

Television footage of the ship stranded on the Astrolabe Reef, 22 kilometres (15 miles) off the North Island coast, showed a gaping crack in its hull, with Transport Minister Steven Joyce saying saying it would "probably" shear in two.

Up to 300 tonnes of heavy fuel has already leaked into the environmentally sensitive Bay of Plenty since the Rena ploughed into the reef last Wednesday, creating New Zealand worst maritime pollution disaster.

Officials have warned the crisis will deepen if the Rena breaks up and releases all 1,700 tonnes of oil it is carrying.

The ship's captain, a Filipino in his 40s, appeared in Tauranga court amid a heavy police presence, charged with operating a vessel in a manner causing unnecessary danger or risk.