Fears grow for stricken NZ ship Salvage experts say they do not know how much longer the vessel at the centre of New Zealand's worst maritime pollution disaster can hold together.

After 10 days grounded on the Astrolabe Reef in the Bay of Plenty, the container ship Rena is still carrying over 1,000 tonnes of heavy fuel oil and diesel.

Maritime New Zealand salvage coordinator, Bruce Anderson, says there are indications that the oil leak has been stemmed.

Large cracks have opened up on both sides of the ship's hull and there have been concerns the ship's stern may break away.

Now Mr Anderson says new information shows the ship's stern is less supported by the reef than first thought.

"Divers have indicated the stern is in a precarious situation as a portion of the vessel is still floating in deeper water, while the front of the vessel is firmly grounded on the reef," he said.

"So it's not as stable as we had hoped."

Salvage crews working on board are expected to start pumping oil to a nearby tanker on Sunday.   (more, including new pic)