Maritime New Zealand says the longer the container ship Rena is subject to the weather, the weaker the wreck will become.
The broken-off stern section of the container ship shifted in the rough weather in Bay of Plenty.
And Maritime New Zealand's Rena recovery and response manager David Billington said Thursday the wreck was structurally damaged.
He says it is not known exactly how many containers have spilled from the ship but two have already been located.
Maritime New Zealand issued a navigation warning after the containers broke free.
It says there is also still potential for oil from the Rena to wash ashore.
Dark patches of oil have been seen around the wreck and a light oil sheen is stretching just under one nautical mile away.
The 47,000-tonne container ship hit the Astrolabe Reef near Tauranga on 5 October last year, spilling containers and oil.
In January, the ship broke in two with the stern section partially sinking and the bow remaining on top of the reef.
Maritime New Zealand spokesperson Ross Henderson, says some containers spilled into the sea overnight on Wednesday but the number is not known.
He says container debris including bags of milk powder and timber is stretching two nautical miles east south east of the wreck.
Mr Henderson says salvors and Maritime New Zealand are monitoring the situation closely.
Salvage efforts have recently been focussed on removing the remaining estimated 30 tonnes of heavy fuel oil from the Rena.
Almost half of the ship's 1300 containers are onshore, with 575 removed by salvors and 72 recovered from the water and shoreline.
