The Rena was carrying 1368 containers when it crashed into Astrolabe Reef last week. The cargo ranged from timber and wool to frozen food, animal pelts and a Ford Mustang car.
The Port of Tauranga kept shipping channels open yesterday, despite Maritime Union of New Zealand's Joe Fleetwood comparing the risk to ships to that of an iceberg.
"If a yacht or small pleasure craft struck one [a container] they would disintegrate. A big ship hitting it, it has the potential to hole it."
Tugs belonging to salvage company Svitzer could be seen corralling about 15 containers floating amid hundreds of pieces of debris in a huge oil slick.
Some had washed ashore at Motiti Island and broken against the rocks, spilling their contents into the sea.
It was confirmed that one of the drifting containers carried alkysulphonic liquid, but authorities did not know where it was. The chemical is classed as hazardous but is water-soluble. It is not considered a significant health risk but authorities said it could cause "localised effects" to the seabed.
Ten containers carried other hazardous substances.
An aerial survey was being done to reassess the number of drifting containers. The specialist cargo ship Polcoma, equipped with a heavy lifting crane, was due to start lifting the containers from the water on to its deck.