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

With the nation's meteorological service forecasting deteriorating conditions, including possible gale-force winds, from Monday afternoon, the race to remove heavy fuel oil from the 47,000-ton 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 tons of oil is still on board.

The crippled vessel has already spewed an estimated 20 tons of oil into the bay, creating a five-kilometer oil slick and killing a number of seabirds caught in the toxic sludge.

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

Key, who flew over the accident scene 22 km off the coast of Tauranga on Sunday, said two government probes had been launched into how the Rena ran aground on the reef in calm conditions.

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.

Maritime New Zealand (MNZ) said the operation was making "good progress on a number of fronts" as night fell on 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," a process MNZ said would take 2-3 hours.

The ship'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.