A stricken ship wedged on a reef off the coast of Tauranga is continuing to list and has now leaked up to 300 tonnes of oil.
Environment Minister Nick Smith says the grounding of the Rena is the most significant maritime environmental disaster in New Zealand to date.
Maritime New Zealand says between 200 and 300 tonnes of oil has now spilled from the container ship, which has been wedged on Astrolabe reef, 12 nautical miles off Tauranga, since 5 October.
The latest spill came from a main fuel tank which ruptured when the vessel moved in heavy seas on the reef early on Tuesday, damaging its keel about halfway along its length.
The ship continues to list, and on Tuesday night was leaning 18 degrees to starboard in swells of four to five metres. In the morning, it had been listing at between three and five degrees.
However, a salvage expert says there has been no change in its structure and he is confident the Rena will stay in one piece at this stage.
The 47,000-tonne ship was heading towards Tauranga from Napier when it ran aground carrying 1700 tonnes of fuel and 11 containers of dangerous goods. Four contain the hazardous substance ferro-silicon which can be explosive if exposed to water.
Maritime New Zealand told a public meeting in Tauranga on Tuesday night that one of the containers in the hold with ferro-silicon in it has been submerged. It says tests show nothing has leaked from the container.
Fears for environment
Oil has already washed up on Bay of Plenty beaches and Motiti Island, about 7km from the ship.
Maritime New Zealand says very strong currents mean it is likely more oil will wash up on the coast on Wednesday and the cleanup is likely to take weeks. At least 50 dead birds have been recovered so far.
Environment Minister Nick Smith said on Tuesday the disaster is likely to get worse.
"The advice I am receiving is that the amount of oil that's been released over the last 24 hours is five-fold what was released in the earlier periods, and that the situation over the coming days from an environmental perspective is going to get significantly worse."
The previous biggest oil spill was from the Jody F Millennium which leaked 25 tonnes of oil when it ran aground off Gisborne in 2002.
Earlier on Tuesday, all crew and salvage teams were evacuated from the Rena as a precaution following a mayday call due to rough seas. One naval crew member was moderately injured.
Maritime New Zealand's salvage unit manager Bruce Anderson says it is unclear how much more oil could come from the ship, as it is not known how badly damaged the fuel tanks are.
There has also been further damage to the bow, causing more flooding in the forward holds. Salvage experts will return to assess the state of the tanks once bad weather clears.
A gale warning is in force for Bay of Plenty with very rough seas forecast, as well as north-east winds rising to 35 knots by Wednesday afternoon. Conditions are expected to ease on Thursday.
The top priority remains getting oil pumped off the ship before it can be salvaged. Draining the oil is at least a 48-hour process and hundreds of containers will then have to be removed. The ship's containers are sealed for travel at sea, but are being closely monitored.
Information from the Australian Maritime Safety Authority shows the Rena was detained in Fremantle in July this year due to problems with hatch covers and cargo stowage. Checks were made in New Zealand to make sure those issues had been rectified.
Inflatable barge company offered help
A manufacturer of inflatable oil recovery barges says he could have recovered most if not all of the oil from the Rena, but Maritime New Zealand never responded to his offer for help.
Ronald Winstone from Lancer Industries in Auckland says the 100-tonne barges the company supplies to 30 countries would have been ideal to get into place quickly before the weather turned bad.
Mr Winton says the barges are inflatable and can rest alongside stranded ships even in two-metre swells.
He says he offered Maritime New Zealand the use of two of his barges to add to the two it already owns, but received no response.