New Zealand's Prime Minister John Key says "serious questions" must be answered about why a container ship ran aground on a reef off one of the country's most spectacular coastlines.
Oil leaking from the Liberian-flagged Rena has created a 5-km (3-mile) slick.
An all-out effort is under way to remove nearly 2,000 tonnes of oil from the vessel, which is stranded 12 nautical miles off the coast.
Heavy swells and gale-force winds are forecast for the area from Monday.
Officials say 20-30 tonnes of oil have spilled into the Bay of Plenty, one of the country's top tourist destinations, since the MV Rena ran aground on Wednesday.
If the ship breaks up, it could release 1,700 tonnes of heavy fuel into an area that is home to whales, dolphins, seals, penguins and a variety of other birds.
John Key, who flew over the scene in a helicopter on Sunday, said two inquiries to determine why the ship had collided with the Astrolabe Reef were already under way.
"People know about the reef, and for it to plough into it for no particular reason - at night, in calm waters - tells you something terrible has gone wrong and we need to understand why," he told Radio New Zealand.
New Zealand's oil spill response agency, Maritime New Zealand (MNZ), said the salvage operation was making good progress, with "significant equipment" - including a naval supply ship, two in-shore patrol vessels and two helicopters - now at the scene.
A barge is berthed alongside the Rena and the salvage team hope to begin pumping oil off the stricken vessel in the coming hours, MNZ said in an update on Sunday.
Salvage experts are on board the Rena and have specialist equipment to warn them if the ship is in danger of breaking up, it added.
"The top priority is to first remove the oil, then lighten the vessel by removing the containers, and finally, move the ship off the reef," the MNZ said.
Two barges have begun scooping up the oil that has spilled into the sea, but the MNZ said it expected to see more oil in the water in the coming days and washing up on nearby beaches around Wednesday or Thursday.
"The weather is starting to change, and is forecast to deteriorate over the coming days, which will impact on both the salvage and oil recovery effort," it warned.
Experts say the arrival of stormy weather could lead to the break-up of the Rena, as one end is stuck on the reef while the other end floats free.
"Once you increase the swell, the upsurge, the lift on the aft end of the ship will increase, and the chances of her separating start to rise," explained Marine Risk Assessor John Riding.
The MV Rena's owners, Greece-based Costamare Inc, said they were "working tirelessly" on the response.
"Minimising any impact to New Zealand's coastline is the absolute priority," the firm said in a statement. "The current primary focus of the salvage operations is the safe transfer of the vessel's fuel oil from her tanks."
The next 48 hours will determine whether this incident can be contained or become a serious environmental disaster, the BBC's Duncan Kennedy in Australia says.
The department of conservation has established two wildlife rescue centres and dispatched teams to scour the beaches and islands of the Bay of Plenty looking for oil-covered animals and birds.
The maritime authorities has said a total of eight oiled birds, including little blue penguins, had been recovered and taken to a wildlife facility in Te Maunga.
"From tip to toe, they are covered in black sticky gunk, matting up all their feathers right down to the skin," said Brett Gartrell of New Zealand's Wildlife Health Centre. "They have ingested it and started to get anaemic, which is part of the toxic effect of the oil."
MNZ has established a one-kilometre maritime exclusion zone around the ship and warns that the fuel oil is toxic.
The animal welfare group Forest and Bird said the timing of the accident, in the middle of the breeding season for birds, was "disastrous".