Oil Spill: Salvors board Rena

No oil will be pumped off Rena today, and salvors say the "grinding and groaning" ship is more unstable than originally thought.

Cargo vessel Rena ground to a halt on Astrolabe Reef last week, and oil leaking from the cracking ship has killed nearly 1000 birds and contaminated beaches.

Seven salvors and two industrial chemists were helicoptered on to the ship today, but could not start pumping the estimated 1,346 tonnes of oil on board the vessel.

Maritime New Zealand (MNZ) salvage head Bruce Anderson said workers have been able to open the first man-hole to a fuel tank and were testing to see if it was safe to enter on the unstable ship.

"The stern is in a precarious situation. It's not quite as stable as we once thought."

Divers have been inspecting the port and starboard side of the vessel over the last two days, he said.

"The message that they're sending back to us is that the vessel is sitting with a portion of it still in space, so it's not as stable as we had hoped."

He said if the weather turned, there was a chance 100 tonnes of oil in the duct keel could leak out.

Mr Anderson said a salvage expert deemed the ship one of the worst wreck he'd ever seen.

"This thing was grinding and groaning away as the vessel was twisting and mashing parts of it up. He was saying it's one of the scariest he's seen."

MNZ scene commander Nick Quinn said the oil extraction process would be in "difficult and potentially hazardous conditions," as the ship is still on a 21 degree list.

Four platforms had been attached to the side of the ship to hold welders, power packs, generators and machinery to complete the oil extraction.

An Archimedes screw pump will be inserted into the tank to extract the oil hardened to a consistency of "marmite", he said.

The salvors were to be helicoptered off the boat today and would try to return tomorrow, he said. .