The cargo ship that has been grounded on a reef off New Zealand's North Island for more than three months has broken clearly in two after being battered by waves of up to seven meters over the weekend.

Cargo containers have begun spilling out of the inside of the Liberian-flagged Rena and New Zealand shipping authorities warned Sunday of the possibility of more oil pollution.

The two pieces of the 47,000-tonne vessel were 20 to 30 meters apart, but still grounded on the Astrolabe Reef, about 12 nautical miles offshore near the port of Tauranga, said Maritime New Zealand (MNZ) Sunday.

The forward section of the ship was still firmly wedged and the aft section had separated and moved about 13 degrees to starboard.

MNZ salvage unit manager David Billington said the new damage to the ship had resulted in the loss of a large number of containers and debris.

"While the two sections of the Rena currently remain on the reef, there's no question the ship is badly damaged with the severe movement breaking off many of the hatch covers and releasing containers from the holds," Billington said in a statement.

Salvors were assessing the state of the vessel so that naval architects could try to determine its stability.

At least 23 containers were confirmed as lost from the ship and were floating or partly submerged, while another seven were thought to be in the water.

Billington said more were likely to be lost and the vessel was surrounded by a large debris trail, including wood.

Container recovery company Bramear Howells had tugs en route to tag containers with buoys because it was too rough to tow or safely recover them, while vessels with trawl nets would be sent out to collect debris once weather conditions improved.

Navigational warnings had also been issued to shipping, and shipping lanes were being monitored for containers and debris.

MNZ national on-scene commander Alex van Wijngaarden said the National Response Team had been mobilized, which included trained oil spill response and wildlife experts, who were preparing for the likelihood of more oil coming ashore.

"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," said van Wijngaarden in the statement.

"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. The wildlife response had also been increased to help deal with any affected wildlife."

Oil was expected to begin washing ashore about midnight Sunday and large amounts of debris and containers could also hit the beaches.

Two senior officers from the ship's Filipino crew have been charged in connection with the grounding on Oct. 5 last year and the resulting pollution.