Bad weather could break up a cargo ship that ran aground on a reef off the North Island of New Zealand more than seven weeks ago, the country's shipping authority said Thursday.

Strong winds and "boiling" (rough) seas around the port side of the Liberian-flagged Rena were increasing stress on the vessel, said Maritime New Zealand (MNZ) in a release.

"The salvage team are keeping a very close eye on the Rena in light of today's rough weather conditions. While there has been no change in the integrity of the vessel, it remains in a fragile condition, and is at the mercy of the weather," said MNZ salvage unit manager Arthur Jobard.

There were no reports of fresh oil leaks overnight, but cleaning operations would continue in some areas on Thursday.

A salvage remained connected to the Rena and salvors were on standby for the weather to improve while monitoring its condition.

For safety reasons, nobody was aboard the Rena, Jobard said.

As of late Wednesday, 95 of the 1,280 containers aboard been removed from Rena, with another two recovered in pieces.

The total number of containers lost overboard was 87, following the discovery of the wreckage of two containers on Wednesday. The total number previously unaccounted for was 54, according to container recovery specialists Braemar Howells.

A large team of oil spill responders remain alert and ready to respond should more oil be released from the Rena during the current unpredictable weather conditions, said national on-scene commander Alex van Wijngaarden in the statement.

Last week, MNZ announced that most of the beaches in the Bay of Plenty region had reopened to the public.

Earlier this month, salvors finished pumping out the bulk of the heavy fuel oil from the Rena's tanks, enabling them to begin removing the cargo containers.

Earlier this week, wildlife experts began the release of birds that had been cleaned and treated after being covered in oil from the ship.