The Liberian-flagged, Greek-owned Rena has been stuck on the Astrolabe reef in the Bay of Plenty since October. Late Saturday heavy swells began battering the ship and the bad conditions were expected to last several days, Maritime New Zealand spokesman Ross Henderson said.
He said there had been a "significant discharge" of hundreds of steel containers into the sea.
"This may result in the 3-mile exclusion zone being extended," he said. "The National Response Team has been activated to respond to the potential release of oil from the ship and to treat any affected wildlife.''
More than 1,000 tons of oil have been removed from the ship since the grounding, but at least another 300 tons remain aboard, officials said.
Despite the oil recovery efforts, at least 20,000 seabirds have been killed in the area and local media describe the grounding as New Zealand's worst maritime environmental disaster.
As for debris, many containers containing aluminum sank, although others were floating, prompting maritime hazard warnings. Some of the debris was expected to begin washing ashore in coming days and citizens were asked to report any discoveries from the wreck, the report said.
The ship's Filipino captain and navigation officer will appear in court Feb. 29 to face charges.