The MV Rena struck a reef in the Bay of Plenty in October, dumping around 2,000 barrels of oil into the sea. Maritime New Zealand, the agency responding to the disaster, managed to get most of the oil out of the stricken vessel by December but residual oil remains.
MNZ said a wave more than 39 feet high was reported on the reef surrounding Rena and seas of around 25 feet were reported Wednesday. A sheen of oil was seen stretching about a half mile from the site of the wreck.
There were no reports of oil reaching the shores and MNZ said no wildlife was affected.
The ship's master and second officer pleaded guilty to 10 of 11 charges related to the October accident.
The most serious charge, attempting to obstruct justice by altering ship's documents before the grounding, carries a maximum penalty of seven years in prison. The Rena's master faces four of those charges while the second officer faces three.
They are to be sentenced May 25.