The New Zealand navy was called in 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 300 defence personnel were on standby while a further 200 people, including specialists from Australia, Britain, Holland and Singapore, were part of the team hoping to contain the five-kilometre (three mile) oil slick.

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 early 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.

Some new oil was spotted later Saturday but this appeared to have dispersed.

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, 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.

The salvage operation is complex because the vessel is in the unique situation of having one end stuck hard on the reef while the other half of the ship was floating, officials said.

MNZ's salvage adviser Captain Jon Walker said an international team was working on a salvage plan.

"I've worked with these people, they are the best," Captain Walker said, as he admitted that the operation would be difficult because the ship is listing at an 11-degree angle and the decks are covered in containers.

With no cranes on the ship, specialist heavy lifting equipment will also have to be brought to the vessel to remove the cargo.

As teams of naval architects are working around the clock to assess the strength of the ship, salvors are looking at how to safely refloat the vessel once the oil is removed and the ship lightened of some of its containers.

Officials hope the salvage team can start pumping oil on Sunday, but this depends on the damage to the ship and the prevailing weather conditions.

The weather is expected to remain fine on Sunday but winds are expected to build, making the salvage difficult, from early next week.

Prime Minister John Key will visit the accident site on Sunday.