Maritime New Zealand is reporting Rena has suffered substantial structural failure with a crack appearing in the number three cargo hold on the starboard side.

Rena, stranded on Astrolabe Reef. Photo: NZDF.

This has been caused by the movement of the vessel as the stern, which has remained afloat, shifting with the waves, while the front part of the ship remains stuck on the reef.

There is a concern that the stern of the vessel may break away.

The salvors have three tugs mobilised either to hold the stern on the reef while further effort is made to remove the oil, or to tow the stern to shallow water where they will remove the oil.

Naval architects are working on possible scenarios.

Iwi are also involved in advising on any cultural issues regarding moving or sinking the ship.

A number of containers have now come off the vessel. Those remaining continue to move, making it extremely dangerous for salvage crews to work on board.

Six vessels have been mobilised to intercept the drifting debris in the water.

MNZ national on scene commander Nick Quinn is confident that he has the people, equipment and plans to cope with the increasing scale of the Rena response.

'Our experience means we have been preparing for a worst case scenario right from the start.

'We already have hundreds of well trained responders from a number of organisations across land, sea and air operations, and have access to more if we need them.

'Our priority is the here and now, and cleaning up the oil, however, this is not a quick fix so we are here for the long haul.

'Until now we have had a light oiling of beaches – this will significantly increase as more oil washes ashore over the coming days.

'We are continuing our plans for getting people onto the beach for the massive cleanup task.”