Salvage crews were scrambling to off-load oil from a stranded container ship in New Zealand's pristine Bay of Plenty Sunday, as Prime Minister John Key demanded to know why the vessel hit a reef in calm waters.

With the official Metservice forecasting deteriorating conditions, including possible gale-force winds, from Monday afternoon, the race to remove heavy fuel oil from the 47,000-tonne vessel Rena took on added urgency.

Officials fear the stricken ship will break up and sink in the North Island bay and potentially cause New Zealand's worst maritime pollution disaster in decades if the 1,700 tonnes of oil is still on board.

The crippled vessel has already leeched an estimated 20 tonnes of oil into the bay, creating a five-kilometre (three mile) oil slick and killing a number of seabirds caught in the toxic sludge.

It could reach land as early as Wednesday blighting one of the nation's most spectacular coastlines.

Key, who flew over the accident scene 12 nautical miles (22 kilometres) off the coast of Tauranga Sunday, said two government probes had been launched into how the Rena ran aground on the reef in calm conditions early last Wednesday.

The accident -- which occurred in a wildlife-rich area that is home to whales, dolphins, penguins and seals -- seemed inexplicable, Key said.

"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 reporters.

His visit came as two barges began scooping up spilled oil, the first time response teams have been able to get out on the water and attack the slick.

Previously, they had been limited to spraying chemical dispersants from aircraft and helping affected wildlife as they waited for specialist equipment to arrive from elsewhere in New Zealand and Australia.

The New Zealand navy also had two ships on the bay testing equipment designed to contain the oil slick, Maritime New Zealand (MNZ) said.

MNZ on-site controller Rob Service said an Auckland-based tanker normally used to refuel cruise liners had arrived, and salvage crews hoped to begin pumping the heavy fuel aboard the Rena onto it later Sunday.

The Rena's owner, Greece-based Costamare Inc., said all involved were "working tirelessly" on the emergency response.

"Minimising any impact to New Zealand's coastline is the absolute priority for Costamare Inc. and the current primary focus of the salvage operations is the safe transfer of the vessel's fuel oil from her tanks," it said in a statement.

While the Liberian-flagged vessel was badly damaged when it hit the reef, Costamare said "evaluations so far indicate that hull stresses are within allowable limits and that there is no deterioration of the ship's condition".

Officials hope removing the oil will help efforts to refloat the ship.

However, they describe the salvage operation as complex because the vessel is in the unique situation of having one end stuck on the reef while the other end is still floating.

Toxic discharge has already killed a number of seabirds, with six Little Blue penguins and two shags receiving treatment at wildlife rescue centres after being found coated with oil, MNZ said.

Conservation Minister Kate Wilkinson said extra animal carers had been placed on standby for further arrivals.

"Let's hope we don't get them, but I flew over the vessel this morning and there's quite a lot of oil out there," she added.

Teams were scouring the Bay of Plenty's beaches and islands for any other affected wildlife but indications so far were that none of the five-kilometre (three mile) oil slick had blown ashore, it added.

Some 200 people, including specialists from Australia, Britain, Holland and Singapore, have joined the oil slick response team, with 300 defence personnel on standby if needed for shoreline clean up work.

New Zealand health authorities on Sunday warned people not to eat seafood from the bay until further notice.

"Any seafood that has off or petrol-like odours should be avoided," it said.