Salvage crews are fighting to off-load oil from a stranded container ship in New Zealand's pristine Bay of Plenty, as the country's prime minister John Key demands to know why the vessel hit a reef in calm waters.

With the weather bureau 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 has taken on added urgency.

Officials fear the stricken ship will break up and sink in the North Island bay, potentially causing 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 spewed an estimated 20 tonnes of oil into the bay, creating a five-kilometre oil slick and killing a number of seabirds caught in the toxic sludge.

Officials say it will take at least two days to empty Rena's tanks and the slick could reach land as early as Wednesday, blighting one of the nation's most spectacular coastlines.

Mr Key, who flew over the accident scene 22 kilometres off the coast of Tauranga on Sunday, says two government investigations have been launched into how the Rena ran aground on the reef in calm conditions early last Wednesday.

He says the accident - which occurred in a wildlife-rich area that is home to whales, dolphins, penguins and seals - seems inexplicable.

"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," Mr Key said.

The prime minister's 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.

Maritime New Zealand (MNZ) says the operation is making "good progress on a number of fronts" as night fell Sunday, with a tanker normally used to refuel cruise liners berthed alongside Rena and preparing to offload her oil.

"The salvage team will begin pumping oil as soon as the equipment is connected and tested," MNZ said, adding the process would take two to three hours.

Rena's containers had been secured with extra lashing as a precaution and MNZ said officials would work "around the clock to remove the oil" onto the tanker, the Awanuia, before the worst of the weather hit.

"It is expected to take about two days to remove oil onto the Awanuia, all things going well," MNZ said.

Crews clean oil from a sea bird ( AFP )

Experts boarded the troubled ship and fitted it with sensors to monitor stress to the hull, while all vents had been sealed to prevent further oil from escaping.

Scooping and chemical dispersing will continue, with containment not considered a viable option due to the water's depth and strong waves and currents near the ship.

A shoreline clean-up is the "least preferred option", requiring significant time and resources, but MNZ says it is impossible to stop oil reaching the coast and it is "fully prepared" for that eventuality.

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

While the Liberian-flagged vessel was badly damaged when it hit the reef, Costamare says "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 - a complex task because half of it is lodged on the reef but the other end is still afloat.

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 have been scouring the Bay of Plenty's beaches and islands for any other affected wildlife but indications so far are that none of the slick had blown ashore.

It is estimated there are about 200 birds in the slick's path.

Some 250 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.

AFP