The captain and second officer of a cargo ship that ran aground on a New Zealand reef last year have pleaded guilty to mishandling the ship and trying to cover up their responsibility for the environmental disaster.

The unidentified suspects, both Philippine nationals, were in charge of the Liberian-flagged Rena when it tore into a reef in the North Island's pristine Bay of Plenty October 5.

The wreck dumped 400 tons of fuel oil and killed thousands of fish and seabirds. New Zealand authorities call it the country's worst maritime disaster.

Both defendants pleaded guilty Wednesday of charges of operating the ship in a dangerous manner and altering ship documents. They also pleaded guilty to discharging dangerous substances.

They will be sentenced on May 25 when the court will publicly identify the suspects.