Police say a canister of rat poison found on a NSW beach is one of a batch that has been washing up on the Queensland coast since February last year.
The 30cm high metal canisters contain toxic aluminium phosphide, which can be fatal if inhaled or ingested.
A man walking along Salt Beach at Kingscliff, on NSW's far north coast, found one of the containers covered in barnacles and lying on the sand on Friday.
A Hazmat team from Fire and Rescue NSW examined its contents, and police say it is the same as others found on beaches in central Queensland.
Based on the barnacle growth on the canisters, it is believe they have been in the water for some time.
"We believe it's related to the ones washing up in Queensland," Byron Bay Inspector Greg Jago told AAP on Friday.
"It appears they're coming down with the currents."
He warned people to be on the lookout for more canisters and not to handle them but alert authorities.
"We believe the contents are toxic and there is a significant potential risk to anyone who tries to open them.
Inquiries by NSW Police have also revealed that the same canisters have been spotted on beaches in the United States.
Queensland authorities have received reports of the containers being found between Lady Elliot Island and Mabuiag Island, with the most recent being discovered at Zilzie near Rockhampton.
No one seems sure where they are coming from.
Even mild exposure of their contents by inhalation could cause a ringing in the ears, fatigue, nausea and pressure in the chest.