Maritime New Zealand says eight containers from the cargo ship Rena fell off the wreck during rough weather in Bay of Plenty last week and only one has been located so far.
The ship has been stranded on Astrolabe Reef, off the coast of Tauranga since 5 October last year, and sustained further structural damage during the rough weather.
One container that fell into the sea was found by environmental clean-up company Braemar Howells, but it sank and now has a buoy attached so it can be recovered.
Maritime New Zealand says the other containers are being sought by sonar, but it is presumed they have also sunk.
Meanwhile, more debris from the Rena has been removed from Waihi Beach, Tuapiro Point near Katikati and Onemana Bay north of Whangamata.
A clean-up of milk powder and timber that washed ashore at Motiti Island after the storm is expected to be completed early this week.