Several small pockets of oil have been found inside the wreckage of the MV Rena, off the coast of Tauranga.
They were discovered in the remains of the engine room and accommodation block by divers, who were investigating small amounts of oil on the water's surface.
A marine consultant working on the cleanup, Roger King said the oil was at depths of between 35 and 48 metres, and careful planning would be needed to recover it.
He said the quantity was believed to be less than a tonne.
Maritime New Zealand said it was supportive of the action being taken.
The container ship ran aground on the Astrolabe Reef in 2011, causing hundreds of tonnes of fuel oil to spill into the sea.