The New Zealand navy was called in on Saturday to help clean up an oil slick in the pristine Bay of Plenty that leaked from a stranded container ship which now threatens to break apart on an offshore reef.
The navy had deployed four ships to assist efforts to contain pollution from the 47,000 tonne container ship "Rena", which hit a reef off the North Island town of Tauranga earlier this week, Maritime New Zealand (MNZ) said.
It said 500 defence personnel were also on standby for shoreline clean up work if the five-kilometre (three miles) long oil slick reached the coast.
The toxic discharge has already killed a number of seabirds, while five Little Blue Penguins and two shags were being treated after being found coated with oil on the Bay of Plenty's beaches and islands, MNZ said.
The government has warned the accident could become the country's worst maritime pollution disaster in decades if the "Rena" sinks on the reef.
The massive bay at the top of the North Island is regarded as one of New Zealand's environmental jewels. It contains two marine reserves and is home to whales, dolphins, seals and penguins.
The reef which the "Rena" struck is 22 kilometres offshore and MNZ said that while the oil slick had not yet reached the coast, computer modelling showed this was possible in coming days.
MNZ on-site controller Rob Service said a monitoring flight on Saturday showed oil appeared to have stopped leaking from the stricken vessel and much of the slick had been reduced to a "sheen" of thinly-spread oil.
"Obviously from our perspective this is good -- we will be continuing to monitor the slick," he said.
But the problem of dealing with the 1,700 tonnes of heavy fuel oil on board the ship remains. A worst-case scenario would see the 21-year-old vessel, which is already badly damaged, sink on the reef, spewing the oil into the sea.
With the weather forecast to deteriorate next week, MNZ on-site controller Rob Service said removing the oil from the stricken vessel was the top priority.
He said caps were being placed on the ship's fuel tanks to try to prevent the oil leeching out even if it sank.
Service said criticism that the fuel transfer was taking too long was uninformed.
"This is not like removing fuel from a dinghy," he said.
"It's not even like removing fuel from a 30-metre (100-foot) fishing vessel. We're talking about an extensively damaged 236 metre-cargo vessel -- this is a challenging and complex operation."
He said a state-of-the-art tanker normally used to refuel visiting cruise liners was steaming to Tauranga from Auckland to help salvage the "Rena".
Service said chemical dispersants were ineffective on an oil slick that was so thinly spread but they would be used if there was another spill of the thick sludge seen in the days immediately after the accident.
The heavy swells so far off the coast also mean the type of floating booms available in New Zealand cannot be used to contain the spill.
Service said MNZ had sent for 1,250-metre long offshore booms from Australia, along with three heavy-duty skimmers that can scoop oil from the sea.
Prime Minister John Key will visit the accident site on Sunday.