Maritime New Zealand says it will be a painstaking and dangerous process to remove the 1260 containers still on the Rena.
The cargo ship ran aground on the Astrolabe Reef on 5 October. Eighty eight containers have since gone into the sea.
Salvors have removed all but about 50 tonnes of heavy fuel oil from the ship and are now looking at using a crane barge to take off the containers.
The agency says their removal will be complex and the crane barge might only be able to remove five containers per day to begin with.