Bay of Plenty residents are being warned not to touch any canisters of a dangerous fumigant washed overboard from a cargo ship off the coast.

The MV Gladiator was heading to China from Napier in rough seas last Wednesday night when a box containing 14 flasks of aluminium phosphide washed overboard about 200kilometres off the Opotiki coast.

Left: Canisters of the fumigate. Right: The MV Gladiator. Photo: Port of Napier.

Each flask, which contains the chemical used for treating wood products, weighs 1.5kg and has a red and white label.

In a press release issued today Bay of Plenty Regional Council says they were only informed of the incident this morning.

Council Maritime Operations manager Reuben Fraser says the box of canisters had been placed in the ship's storeroom, but the door was left open when the ship sailed, and during rough weather water washed into the storeroom, washing the flasks and other items into the sea.

He says the chemical generates toxic phosphine gas in contact with air or moisture, and could burst into flame.

'This is a highly toxic substance, so people should not try to pick it up or open it,” he said.

'With school holidays this week it's vital that parents warn children playing on the beach not to touch the canisters, since they are small enough to attract children's interest.”

Reuben says there is only a small chance that the canisters entered Bay of Plenty's waters, as prevailing currents and winds were likely to take them further out to sea, however residents need to be aware that they could wash up on Bay beaches.

Anyone finding the 1.5 kg canisters should not open them and report them to the Regional Council's Pollution Hotline 0800 884 883.

People exposed to even low levels of the chemical will feel ill and it can also cause ringing in the ears, fatigue, nausea and pressure in the chest. High concentrations can be fatal. The chemical can irritate skin and eyes and it is toxic if swallowed.

'ALP is safe in the flask and is very difficult to open. If the flasks wash up we will collect and dispose of them,” he said.