Salvage crews are in a desperate rush to off-load oil from a stranded cargo vessel that has spewed 20 tonnes of oil in New Zealand's Bay of Plenty amid forecasts of bad weather, as gale-force winds and oil spill could potentially result in the worst maritime pollution in the country.

NZ weather forecasters have announced possible gale-force winds from Monday afternoon, causing fears of the cargo ship sustaining more damages and sinking in the waters with its oil cargoes.

Maritime New Zealand (MNZ) said on Wednesday last week some fuel from hydraulic pumps on the Rena, a Liberia-flagged 47,000-tonne vessel, had leaked, but its fuel tanks were still intact.

Meanwhile, NZ prime minister John Key asked for an explanation as to how the vessel hit and got stuck in a reef when the waters were calm.

Key pointed out the location is a wildlife-rich area that is home to whales, dolphins, penguins and seals.

Officials fear Rena could bring to NZ history the worst maritime pollution disaster, unless the salvage crews succeed in complete off-loading in time.

The 20 tonnes of oil that Rena has spilled into the bay has so far created a five-kilometre oil slick and killed a number of seabirds caught in the toxic sludge, according to ABC News.

ABC's sources said it will take at least two days to empty Rena's tanks and the slick could reach land by Wednesday, ruining New Zealand's most spectacular coastlines and precious wildlife.

Mr. Key, who flew to witness the scene off Tauranga on Sunday, has been vocal on demanding an explanation for something that should not have happened given the weather condition at the time.

"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.