Maritime New Zealand is investigating why containers holding dangerous goods on board the cargo ship Rena were not declared earlier.
An additional 21 containers on the grounded ship are now known to be carrying hazardous goods, bringing the total to 32.
The containers hold cryolite, a substance that is dangerous to humans in its dry form. The containers are below deck and under water.
The agency was warned about the containers on Tuesday, despite asking for information about dangerous goods straight after the ship's grounding off Tauranga on 5 October.
It is investigating if the delay in reporting is a breach of maritime law.
Maritime New Zealand director Catherine Taylor says it is the responsibility of the shippers or freight forwarders to correctly identify, label and declare dangerous goods.
"We strongly urge any shippers who have goods on the Rena that have not been correctly labelled to advise us.
"In my view, they have a moral, ethical and legal obligation to do so," she says.
The containers carrying the dangerous goods were loaded at Bluff.
Salvage vessels return to Auckland
Two vessels that played key roles in the salvage operation have had a formal welcome back to their home port of Auckland.
Ports of Auckland owns the tug Waka Kume, which held the stricken ship in place, and part-owns the tanker Awanuia, which took out the Rena's oil.
Captain Rick Hunter, who is one of the masters of the Awanuia, says he had doubted the Rena would stay in one piece.
"I was absolutely certain that we would never get all of the oil off until the last few days," he says.
Captain Hunter says a storm could hit at any time and the ship was "hanging on by a thread".
"Something is stronger in that ship than we really realised," he says.