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 then altering navigation records to cover up their responsibility for the massive environmental disaster.

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

The wreck, near the port of Tauranga, dumped some 400 tons of fuel oil and killed thousands of fish and seabirds, in what authorities call New Zealand's worst-ever maritime disaster.

In Tauranga Wednesday, both defendants pleaded guilty to operating the ship in a dangerous manner and later altering ship documents. They also entered guilty pleas to discharging dangerous substances. Sentencing is set for May 25, at which time authorities say the identities of the defendants will be made public.