Fears of a massive oil spill have been raised after a cargo ship split into two off the New Zealand coast, hurling containers and huge amounts of debris into the sea.

The vessel, Greek-owned Rena, ran aground in October 2011. In what has been described as New Zealand's worst maritime environmental disaster, the rear section of the ship broke away overnight under the pressure of pounding waves, according to an Associated Press report. As much as 400 tons of heavy fuel oil remains on board.

A previous fuel spill from the vessel in October 2011 contaminated a big stretch of North Island beaches, leading to the death of nearly 20,000 seabirds.

The broken-away rear section of the vessel is moving significantly in the water pounded by huge waves. Up to 300 of the roughly 880 containers on board the vessel were lost when the ship broke up and most of them have not been traced yet, the agency added.

"While reports at this stage indicate there has not been a significant release of oil, with the Rena in its current fragile state, a further release is likely. While it is unknown at this stage exactly how much oil may be released, teams have been mobilised and will be ready to respond to anything that may come ashore," Alex van Wijngaarden, on-scene commander for the national response team, was quoted by AP as saying.

However, New Zealand's environment minister Nick Smith said the risk for the environment would be much lesser than the one caused by the spill that occurred when the vessel ran aground in October.