Mysterious canisters containing toxic pesticide have been washing up on Australian beaches, posing a serious danger to anyone who comes in contact with them, the Australian Maritime Safety Authority (AMSA) has warned. 
The silver canisters, which are not much bigger than a drink bottle, are filled with aluminium phosphide - rat poison crystals that are hazardous to humans.
When the powder is exposed to moisture in the air it forms phosphine gas, which is also toxic to humans.
The canisters should be airtight but there have been instances where the seals were broken, presenting a serious hazard to anyone in close proximity to the tampered containers, AMSA said.
Hazardous Material teams have dealt with dozens of canisters found by members of the public washed up along the coast of New South Wales, Queensland, South Australia, Tasmania and the Torres Strait since   February 2012.
NSW and QLD fire emergency services have collected over 40 canisters over the last three and a half years.
Manager of AMSA's Marine Environment Pollution Response Jamie Storrie said they suspected the canisters had all come from the same source: a cargo ship that either dumped or lost its cargo somewhere in the Pacific Ocean and did not report it.
Ships effectively use the canisters as rat bombs, dropping them into the vessel's cargo holds, Mr Storrie said.
"If it was an illegal dumping it would be a violation of international law," he said.
Mr Storrie said the canisters have no distinguishing markers that would help AMSA identify their origin.
Phosphine gas has a terrible odour similar to rotting fish or urine, and rapidly dulls the sense of smell of anyone in close proximity.
Exposure can cause acute symptoms including headaches, nausea, vomiting, difficulty breathing, dizziness tightness of the chest, diarrhoea, fluid in the lungs, liver and kidney damage and in severe cases, death.
The inflammable gas can also spontaneously ignite or explode, which could inflict serious burns. Firefighting can be very difficult because of the powder's toxic chemical reaction to water.
"Canisters should not be moved or opened and any sightings should be reported to emergency services on 000," Mr Storrie said.