The new buckling and fracture of Rena's hull is heightening Maritime New Zealand's concern that the 236m container ship stranded on the Astrolabe Reef is going to break up.

MNZ salvage unit manager Bruce Anderson says the fracture in the hull, first noticed by divers at the weekend, is pronounced.

'It looks like someone has ripped it.”

This fracture is in the ship's stern, close to the bridge of the vessel and is occurring where the ship ceases to rest firmly on the reef.

It is weakening the hull's integrity, which is already cracked further forward, making the ship less safe for the salvage team, although four team members were aboard today.

Bruce says the noise aboard the Rena, previously described by him as 'grinding and creaking – it is the sound of a vessel dying”, is now worse owing to two bend points.

'The vessel is clanking and groaning and so that will be amplified now there is this new movement.”

The salvors onboard Rena today were placing sensors to monitor the vessel's damage and positioning.

If the ship breaks, the MNZ plan to use the Go Canopus tug to tow the stern section to shallower water remains in place despite this new fracture in the hull.

'The new fracture does change some of the buoyancy characteristics.”

There are five sites selected for where the tug can attempt to tow the stern section to and which one is selected will depend on prevailing winds and swell conditions at the time.

There is still 358 tonnes of heavy fuel oil aboard Rena in the starboard five tank and it is hoped if the ship breaks, this tank will remain sealed and no oil will leak from it.

Associate Minister of Transport Nathan Guy says the next 24-36 hours are critical with a storm of 35 knot winds and a 5m swell lashing the ship.

He says a major concern is the loss of containers aboard the ship falling into the sea.

There were 1368 containers aboard when the ship first hit the reef on October 5 and in the first storm to follow 88 fell into the sea.

'The difficulty will be if containers fall into the sea in the high seas that are expected.”

He is asking people to phone 0800 OIL SPILL if they see a container washed up on a beach and not to approach it.

Bruce says there are four hazardous goods containers on the deck of the ship, however, as they are empty he says they should float and are not too much of a concern.

Braemar Howells is contracted to handle the salvage of the containers and it has five tugs in place to gather any floating containers.