BARGES began scooping up an oil slick on New Zealand's pristine Bay of Plenty on Sunday, as salvage crews prepared for the delicate task of pumping fuel from a crippled container ship stuck on a reef.

The New Zealand navy also had two ships on the bay testing equipment designed to contain oil that spilled when the 47,000-tonne vessel Rena hit a reef off the North Island on Wednesday, 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 1,700 tonnes of heavy fuel aboard the Rena onto it later Sunday.

The deployment of the two barges is the first time response teams have been able to get out on the water and attack the oil spill, which has already killed a number of seabirds.

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

The government has warned that the accident could become the country's worst maritime pollution disaster in decades if the Rena sinks, spewing oil into the pristine bay, which is home to whales, dolphins, seals and penguins.

With weather conditions expected to deteriorate on Tuesday, the Rena's owners, Greece-based Costamare Inc., said removing the oil from the stricken vessel was the top priority.

"Minimizing 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 hard 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.

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.

Prime Minister John Key is scheduled to visit the area later Sunday.

Originally published as New Zealand oil slick clean-up swings into action