Mr Anderson said the salvage team did not know yet whether the sea conditions would result in the vessel breaking up and it was being closely monitored for any changes in its condition.
The ship was still holding together but it was looking "increasingly precarious'' and salvors were preparing for the worst,'' he said.
Salvors had sealed the tanks to minimise the potential for further oil leaking from the ship and additional sensors had been placed on the hull to monitor the vessel's movement.
There is an estimated 350 tonnes of oil remaining in the starboard tank, which is still intact.
UK-based container recovery specialists Braemar Howells Ltd had been contracted to manage container recovery and had developed an emergency plan if more containers are lost overboard.
Vessels and aircraft were on hand to respond but any recovery efforts would be weather-dependent.
Mr Anderson said there was a strong possibility of more containers being washed overboard.
The tug Go Canopus remained onsite and teams onboard were monitoring the state of Rena.
Go Canopus was attached to the ship and attempts would be made to tow the stern to shallower water if it became detached from the bow.
National On Scene Commander Mick Courtnell said the oil spill response team was ready to respond to any oil coming ashore.
About 150 New Zealand Defence Force personnel were available for clean-up operations if needed tomorrow (Wednesday) and 250 volunteers would be assisting.
Oil spill specialists would be on the beach at first light tomorrow to assess whether any oil had reached the shoreline.
"We will continue to use both our trajectory modelling and our eyes to monitor the movement of any oil released from the ship. At this stage our modelling indicates around a 12-hour window between oil being released and any oil reaching the shoreline,'' he said.
Spilled oil could reach beaches from Papamoa to Maketu Peninsula.
Techniques used by teams to clean oil after the 11 October spill would again be put into place, said Mr Courtnell.
