Salvors onboard the cargo ship Rena are in a race against the clock to pump oil from the ship which has split in two and is resting in a precarious position on the reef.

Maritime New Zealand salvage unit manager Bruce Anderson says northwesterly winds and large sea swells forecast for this evening could cause a another large oil spill and potentially dislodge the ship from the reef.

Rena, stranded on the Astrolabe Reef for 13 days is breaking up.

'The vessel has broken up in two pieces.

'We are expecting the wave height to increase and choppy seas to cause problems.

'If the wave action dislodged the vessel we are into a different set of circumstances.”

Bruce says the ship is in two pieces, with the stern resting in a precarious position on the edge of the reef, while the front of the ship is securely grounded on the reef.

Bruce says if the ship moves off the reef it could puncture a fuel tank causing more oil to leak.

There is estimated to be 1300 tonnes of fuel still onboard the ship and Bruce says people can expect another large oil spill from the duct keel of the ship.

'It is not a matter of ‘if', but ‘when' the oil hits the beaches.”

Evacuation procedures are in place for salvors working on the Rena and Bruce says if needed they could move from the stern of the vessel to the front of the ship.

Two helicopters are also on standby to remove personnel from the vessel if required.

Nine people are onboard the vessel and are planning on working through tonight to pump oil from Rena onto the barge vessel Awanuia.

'Salvors want to stay on board for as long as they can to pump as much oil off the vessel as possible. If they had to evacuate they would seal the tanks.”

Twenty one tonnes of oil was pumped from the ship overnight in what Bruce says is a long, hard process pushing oil with the consistency of marmite through a three inch hose.

Three people worked on Rena overnight in 'dangerous conditions”.

'It's grinding and creaking – it is the sound of a vessel dying.”

New booster pumps being installed today are expected to speed up oil transfer operations.

So far 88 containers have been lost overboard from Rena with more expected to fall from the ship tonight.

'I wouldn't be surprised if we lose more containers tonight.”

National On Scene Commander Nick Quinn says surveillance areas have been extended to include White Island as the oil is expected to head north in the next 24-48 hours.

'Containers have been sighted near White Island.”

Nick says there is no indication of any oil at sea expected to hit Whakatane but a base has been set up there as a precaution.

Maritime New Zealand is asking people not to collect seafood from the Maketu, Mount Maunganui and Matakana Island areas, and where oil has washed ashore.

'There is no intention to stop fishing in areas apart from within the one kilometre exclusion zone around Rena.”

The aerial exclusion zone has been reduced to 1000 feet and three nautical miles.