New Zealand's transport minister says salvage teams are trying to remove oil from the stricken vessel, Rena

New Zealand's navy has joined efforts to contain pollution from a stricken container ship off the North Island that threatens the country's worst ever environmental disaster.

Four naval ships were deployed on Saturday to assist efforts to contain pollution from the Rena, Maritime New Zealand said.

Five hundred defence personnel were also placed on standby for shoreline clean up, the agency said.

A monitoring flight on Saturday suggested that the 47,000-tonne Rena had stopped leaking oil for now, with much of the slick reduced to a thin sheen of oil, Rob Service, the on-site controller for Maritime New Zealand, said.

"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 Bay of Plenty's Astrolabe Reef, spewing the oil into a marine environment that is home to whales, dolphins, seals and penguins.

The Rena hit a reef off the coast of Tauranga earlier this week and has already created an oil slick more than five-kilometres long that has killed a number of sea birds.

It is not known why the ship ran aground in the early hours of Wednesday morning. None of the 25-man crew were injured in the accident.

'Significant slick'

An estimated 100 tonnes of oil have moved away from the ship and into the water, but two days of chemical dispersant usage has reduced the damaged to a thin sheen of oil, instead of bigger clumps seen earlier, Rachel Morton, a reporter for New Zealand's TV3, told Al Jazeera.

Aside from the concers over the vessel breaking up, another real worry is that the containers aboard the ship could be lost in the ocean, Morton said.

"There are 35,000 tonnes of cargo and most of it is safe to be on there," she said.

"But there are a couple of containers which contain ferrosilicon. When that is mixed with water it becomes highly flammable."

Nick Smith, the environment minister, told local media on Friday that the accident had the potential to be New Zealand's "most significant maritime pollution disaster in decades".

Steven Joyce, New Zealand's transport minister, told Al Jazeera he was not confident that an ecological disaster could be prevented.

"I'm not confident because of the state of the ship, and that it's not an oil tanker, so it's not designed to discharge oil," Joyce said.

The weather in the Bay of Plenty is forecast to deteriorate early next week, giving added urgency to efforts to remove the oil in case the ship breaks apart in heavy swells.

Seabirds at risk

Wildlife experts have raised concern for the plight of seabirds caught up in the slick, with rescue teams dispatched to scour beaches for stricken birds.

Maritime New Zealand said two wildlife rescue centres had been set up, but could not confirm reports that seals had been seen covered in oil.

Animal welfare group Forest and Bird said species of marine bird life at risk from the spill included blue penguins, shear-waters, gannets and petrels.

The group's seabird specialist, Karen Baird, said the timing of the accident, in the midst of breeding season, was "disastrous".

"Many of them are sitting on eggs and some of them have got chicks that are starting to hatch, so that's a big worry for us."

Source: Al Jazeera and agencies