A container ship is grounded and leaking oil into New Zealand's pristine Bay of Plenty, with international crews scrambling to limit the environmental damage and refloat the vessel before it breaks up.

The 47,000-tonne Rena ran aground on Astrolabe Reef on Wednesday. An oil leak from the Liberian-flagged freighter has spread over an area of three miles, according to the BBC. There are estimates of 30 tonnes of oil spilled so far out of the 1,700 tonnes that could be dumped into the ocean if the Rena is wrecked in one of New Zealand's most prized areas of natural beauty.

Maritime authorities have said they are treating birds including little blue penguins brought in covered with oil. Animal welfare workers said the disaster had struck in the middle of breeding season for native birds on the bay.

Australia's ABC network said a team of 200 people including specialists from Australia, the UK, the Netherlands and Singapore had been despatched, and that 300 defence personnel were on standby in case the slick reached the North Island coastline.

The New Zealand prime minister, John Key, toured the scene by air and said two inquiries were under way into the cause. "People know about the reef, and for [the ship] 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 Radio New Zealand.

The Rena had 25 crew on board but none were injured, reports said.

New Zealand navy and salvage ships are working to pump off the ship's fuel oil and move it to safety before attempting to free the ship. Dispersants sprayed from the air on to the slick have not worked and bad weather is expected to hamper the containment effort.