New Rena threat emerges | Bay of Plenty News | Local News in Bay of Plenty

Both sections of the Rena remain stuck on the Astrolabe Reef but there are increasing signs the wreck won't stay that way for long.

Salvors have reported movement in the bow section as sea pressure inside the hollowed-out hull forces 50-tonne hatches to "pop off".

And while the stern section remains perched near the top of the reef, there is a possibility that once underwater salvage operations begin it could slide further under.

Maritime New Zealand salvage unit manager David Billington said container retrieval from the bow continues to be the salvors' priority, as they continue to hand-load much of the cargo from the ship.

Only 35 containers remain on the top deck, with an additional 283 still in the hold below deck, but with every container removed the bow becomes lighter and is starting to move.

Sea water rushing into the open hull is adding to the movement, as a pressure process called "hydraulicking" forces below-deck containers and the 50-tonne top-deck hatches to "pop off".

"The salvage teams on board have reported the movement. They have noticed as they're taking weight off, the free parts are moving. This is just a piece of steel - it's been there for three months getting hammered by the weather, it's weakened," Mr Billington said.

The rear section of the ship, while currently stable, has its own hazards.

Salvage divers continue to assess the wreck and the reef around it and Svitzer has ordered a larger remote operating vehicle (ROV) to help scope out the underwater conditions ahead of any possible wreck-retrieval, Mr Billington said.

"The plan moving forward is for Svitzer salvors to remove the wreck ... but I don't know what plan they're coming up with."

One option would be to use "big grabs" with wrecking claws, such as those found at scrap metal yards, to take chunks of steel piece by piece, Mr Billington said.

Another option would be a process "like a bush saw", where two barges with a chain or cable between them tug back and forth across the wreck, sawing it into pieces. But the danger of either option is that the jarring action involved could work the wreck free from its current site on the wreck and allow it to slide much further down the reef, beyond the reach of surface retrieval.

Simon Rickaby, managing director of container and debris recovery crew Braemar Howells, said the reaction in the Bay had been impressive, but stressed there was still a lot of work to go in the salvage and clean-up of the Rena's mess. "Like my father always said: how do you eat an elephant? One bite at a time. And this is a great big elephant.

"You can be proud. It's a phenomenal response that everyone's pulling together."