Just over a year after the Rena struck the Astrolabe Reef in the Bay of Plenty, containers of hazardous substances still lie beneath the water.

The Minister of Transport, Gerry Brownlee, has confirmed the contents of three containers of cryolite, a byproduct of the aluminium smelting process, have been lost at sea.

The location of a further 17 containers with cryolite is not known.

Mr Brownlee said it was not possible to state with certainty how many of these containers remained within the wreck in cargo holds.

Twenty had been stowed below deck in Hold 3, from where the ship was ripped in half during a January storm, while the other had been stowed below deck in another hold.

A risk assessment provided to the Government stated cryolite posed “both an immediate and long-term pollution hazard to marine organisms and plants”. But scientists had advised Maritime New Zealand that a number of factors suggested the risk could be reduced.

Cryolite was only slightly soluble in water and the rate of release was expected to be diminished because of packaging. It broke down to naturally occurring elements – mainly aluminium and fluoride – which became less harmful in seawater because of reactions with other naturally abundant elements.

Maritime New Zealand began an investigation when it found cryolite had been loaded onto the Rena in Bluff without being classified as a dangerous substance.

The agency decided not to prosecute the manufacturer after it alerted authorities to the error and co-operated to address the gap in its processes.

The contents of containers carrying ferrosilicon and one of potassium nitrate are also thought to have been lost at sea.

Another container of 5400kg of trichloroisocyanuric acid was on the seabed next to the wreck in March, but has not been recovered.

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