Fresh oil leaking from the wrecked cargo ship Rena off the New Zealand coast is expected to reach shore overnight, salvage officials said Tuesday.

But they did not believe the contamination would be as serious as when the Rena first ploughed into a reef three months ago causing New Zealand's worst maritime pollution disaster.

The Rena's stern began sinking and leaking more oil Tuesday after being in a precarious position since the vessel broke in two in a fierce storm over the weekend.

About 150 containers tumbled into the sea and many have been washed up with other debris from the ship on the once pristine beaches at the Tauranga resort area on the North Island east coast.

Maritime New Zealand's on-scene commander Alex van Wijngaarden said a "small amount" of oil has flowed from the stern section, along with debris, mostly timber, and a number of containers.

"This was not unexpected. We are prepared, and we will deal with it," he said.

An oil-spill and wildlife response team has already been mobilised after a sheen of oil "about three kilometres (two miles) long by 5-10 metres (16-33 feet) wide" appeared after the ship broke in two, van Wijngaarden said.

Environment Minister Nick Smith said the amount of oil in the latest leak was in "single digits of tonnes".

When the Rena ploughed into Astrolabe Reef 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.

Maritime New Zealand said the bow of the Rena remained stuck on the reef but about 75 percent of the stern had sunk.

Container recovery company Braemar Howells estimated there were about 400 containers in the stern of the Rena and two tugs had been sent to the area to try to contain those that were drifting.

Officials said the containers and other debris in the water made conditions hazardous for shipping and a three nautical mile exclusion zone around the Rena had been put in place.