Most of the stern section of the cargo ship Rena has slipped off the Astrolabe Reef and has sunk into the sea off the north end of New Zealand's North Island, Maritime New Zealand (MNZ) said.

The stern section of the Rena begins to sink.?[Maritime New Zealand]

Heavy seas pounded the vessel on the weekend, breaking her in two on Saturday night. The bow section remains in place on the reef despite heaving swells. Three quarters of the stern section of the 47,000-tonne Rena, holding an estimated 400 cargo containers, was under water and two tugs were at the site to contain drifting containers.

New Zealand shipping authorities Tuesday imposed new navigation restrictions around the wreckage of a Liberian-flagged cargo ship.

The harbormaster for the nearby Bay of Plenty region ordered all vessels to stay at least 200 meters clear of all containers from the Rena and from any salvage vessel.

The temporary exclusion zones were in addition to the existing 3-nautical-mile exclusion zone around the Astrolabe Reef and any part of the Rena, said a statement from the harbormaster's office.

"The on-water situation is evolving and remains hazardous," said the statement.

"The harbormaster urges vessel operators not to go out on to the water unless it is necessary. If the trip is unavoidable, vessels must proceed at slow speed, keep a good lookout and travel through the area in daylight only. The debris field is extensive, its movement is unpredictable and could extend further."

The MNZ said a small amount of oil had spilled from the stern section, along with debris -- mostly timber -- and "a small number " of containers.