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.

Three canisters containing aluminium phosphate that are belived to have fallen off, or were dumped, by a shipping vessel in the Pacific Ocean. Credit:AMSA

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.