WELLINGTON - The rear half of the Liberian-flagged cargo ship that grounded on a reef off the east of New Zealand's North Island six months ago has finally sunk after being battered by rough seas, shipping authorities said Wednesday.

The bow section of the stricken container ship Rena remains above water about 14 nautical miles (22 km) from Tauranga on the east coast of New Zealand's North Island, in this April 4, 2012 handout picture. The remains of the ship are stuck on a rock reef six months after it ran aground, in what authorities say is one of the nation's worst environmental disasters. [Photo/Agencies]

Maritime New Zealand (MNZ) warned vessels to beware of debris, including cargo containers, after the bow section of the Rena sank near the Astrolabe Reef.

MNZ also warned communities on the east of the North Island that the sinking of the aft section would likely result in the loss of more debris and small amounts of oil into the sea.

The debris and oil were likely to come ashore, but the amount of oil released was only "a tiny percentage" of the oil released immediately after the grounding on October 5 last year, said a statement from MNZ.

A light sheen of oil could be seen stretching about 1 km northwest of the wreck, it said.

The forward section of the vessel was holding on the reef, but had suffered further damage to its rear section.

A wave of over 12 meters was recorded at the reef over Tuesday night, with winds of 50 km an hour and seas of 8 to 9 meters experienced Wednesday, said the statement.

Strong winds and rough seas, which were hampering recovery of the debris at sea, were expected to continue for several days.

"Boat skippers and vessel masters are reminded to take extreme care, as the debris from the wreck poses a significant risk to shipping," said the statement.

In February, the master and second officer of the Rena admitted 10 of the 11 charges they faced in connection with the accident.

The identities of the two men were suppressed when they appeared at the Tauranga District Court.

The captain pleaded guilty to operating a vessel in a manner causing unnecessary danger or risk, discharge of harmful substances from ships or offshore installations, and four charges of willfully attempting to pervert the course of justice by altering ships documents.

The second officer admitted operating a vessel in a manner causing unnecessary danger or risk and three charges of willfully attempting to pervert the course of justice by altering ships documents, but entered no plea on a charge of discharge of harmful substances from ships or offshore installations.

Sentencing of both men, whose identities were suppressed by the court, was set for May 25 at the Tauranga District Court.

The Rena spilled hundreds of tonnes of oil into the sea along with cargo containers and their contents, killing an unknown number of New Zealand's native birds and prompted a massive clean- up of oil from beaches around Tauranga.

On January 7, the vessel split in two and salvors are still trying to remove cargo containers from it.