The captain and the navigating officer of a cargo ship that ran aground on a New Zealand reef last year have admitted culpability in causing the country's worst environmental disaster in decades.

Both men pleaded guilty before a New Zealand court on Wednesday to operating a ship in a dangerous manner and trying to pervert the course of justice by changing the ship's documents after the crash.

The captain pleaded guilty to all six charges filed against him, while the navigating officer pleaded guilty to four of the five charges against him and did not enter a plea on the fifth.

The two men, whose identities have been suppressed, have been remanded on bail and will be sentenced on May 25. They face large fines and up to seven years in jail on the most serious charges.

The captain's lawyer declined to comment to The Associated Press news agency, and the navigating officer's lawyer did not immediately return calls.

The men, both from the Philippines, were responsible for the sailing path of the MV Rena on October 5 last year when it ran aground on the well-charted Astrolabe reef near the port of Tauranga.

In the days after the crash, the ship leaked about 300 tonnes of thick, toxic fuel oil into the sea when it hit the reef, killing thousands of sea birds and fouling beaches up to 100km from the reef.

The 236-metre Liberia-flagged vessel split in two in January this year after foundering on the reef for three months.

Salvage teams, who removed more than 1,000 tonnes of oil from the ship after the crash, are continuing to remove containers off the ship and collect debris off beaches and the sea floor.

New Zealand's government this month estimated the costs of the cleanup at 130 million New Zealand dollars ($108m).

Most of the costs have been met by insurers, although taxpayers have paid for some costs.

The ship is owned by Greek-based Costamare and was chartered by the Swiss-based Mediterranean Shipping Company.

Source: Agencies