New Zealand police were forced to shut down a popular beach Monday to prevent looters raiding containers washed ashore from the stricken cargo ship Rena.

The Rena, which had been stuck fast on a reef since October when it caused New Zealand's worst maritime pollution disaster, broke in two in a storm on Sunday to send containers tumbling into the sea.

Although the devastation was not as severe as maritime officials feared, the once-pristine beaches in the North Island resort area of Tauranga were littered with debris, mainly timber, plastics, bags of milk powder and polystyrene.

There were four containers on the beach and Martime New Zealand on-scene commander Alex van Wijngaarden said a sheen of oil "about three kilometres (two miles) long by 5-10 metres (16-33 feet) wide" was visible offshore.

Wildlife response teams said they had only treated four oiled birds since the Rena broke up.

When the ship ran aground on October 5, about 350 tonnes of oil spilled into the sea, killing at least 1,300 birds. An army of volunteers combed the coastline and saved hundreds more.

At a time when the region is packed with summer holidaymakers, police closed Waihi beach for several hours and people were urged to stay out of the water in surrounding areas.

"The expert advice we have received is for people not to approach items washed ashore for health reasons," police sergeant Dave Litton said.

Looters were taking off with what appeared to be milk powder and "we appeal to those people who have taken objects to return them to the beach where they can be managed by decontamination crews".

Clean-up crews were mobilised to remove the debris as Maritime New Zealand said it believed 150 shipping containers were washed overboard, not 300 as originally thought.

About 700 containers remain on the Rena and a spokesman for the salvors, Matthew Watson, told Radio New Zealand that it had become "very, very dangerous and very difficult" to remove them.

Although the bow of the ship remained stuck on the reef, the rear section of the vessel was causing concern as it moved around.