"If the vessel falls off the reef it could puncture a tank. You don't know what's going to happen if it falls off the reef."
Mr Anderson said there was still some oil not contained in fuel tanks inside the Rena which was likely to spill in bad weather this evening.
A Maritime New Zealand spokeswoman said salvage crews were working with a 10 to 12 hour "window of opportunity" to get oil off the vessel.
They were working to attach booster pumps to help them force the oil 160 metres out of the Rena's fuel tanks into the bunker barge Awanuia, she said.
Svitzer salvage crew spokesman Matt Watson told Radio New Zealand it was likely more oil would leak from the vessel tonight.
The Rena was pivoting even in very low seas, and was at risk of breaking up once high seas set in this evening, he said.
"That ship is very very sick. She is fractured. She is broken. She is on her knees.
"That massive gaping crack down the side... it is quite obvious that if strong winds and strong rain come again that vessel could break up."
Mr Watson said the a main fuel tank could be punctured if the vessel broke up - though salvage crews had carried out sealing work.
If the tanks remained intact, a "much more complicated" salvage operation could be carried out by dive crews, he said.
One tank at the stern of the Rena contains about 700 tonnes of oil and a tank at the rear starboard side of the vessel contains another 300 tonnes.
About 350 tonnes of oil has already been spilled from the Rena. Much of it has washed up on Papamoa Beach south of Mt Maunganui.
Pledge of cash for clean up
The Mediterranean Shipping Company has given the Government a commitment to make a financial contribution to help with the oil clean-up operation.
The company, which chartered the Rena from Costamare, met with Transport Minister Steven Joyce this morning.
The company's view is that they were not legally liable, but Mr Joyce said that was an ongoing issue.
"They gave the assurance that they were very concerned and upset about what happened, that they considered themselves a longstanding member of the New Zealand community," Mr Joyce said after the meeting.
"And they also gave a commitment that they would assist with the clean-up ... My understanding is they will make some financial contribution, and the amount is for them to assess in terms of their reputation in this country and the way New Zealanders feel about what has happened.
"I've certainly encouraged them to think very carefully about that ... New Zealand companies and people they deal with would want to see them stepping up."
Mediterranean runs an active business here and conducts about 100 port calls a year throughout New Zealand.
"We didn't talk about legal liabilities," Mr Joyce said.
"The liabilities will be determined by the lawyers. It's all wrapped up in the definitions in the Maritime Transport Act and that ... will sort itself out in due course."
Mr Joyce said the clean-up job had so far cost about $4m, and he would not be surprised if the final bill exceeded the $12.1m liability cover - depending on how much more oil leaks.
"I would expect further spills to occur at different points of this exercise, so we've got a way to go yet."
He said there may be other avenues to recover costs.
"Both the total amount and who recovers what and where is still up for discussion.
"In the wider context of the Crown accounts, it's not going to have a significant impact at this point. But it's way too early to speculate on those sorts of details."
Scout for coastal oil continues
Meanwhile clean-up crews will continue to look for oil along the region's coastline today.
"We will be focusing on scientific monitoring and surveillance of further patches of oil that come ashore, and will continue to send teams out to pick up oil as needed," national on scene Commander Nick Quinn said.
As of last night, 181 live birds were being treated at the Oiled Wildlife Recovery Centre.
Three fur seals are also in care. About 1,250 dead birds have been recovered to date.
Most beaches remain closed and people are urged not to collect or eat any shellfish in the region. While the main beach at Mt Maunganui is open down to Leisure Island, there is no swimming at all beaches, Mr Quinn said.
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