New South Wales authorities have issued a warning after containers that may have a toxic substance inside them washed up on the state's beaches.

A blue 20-litre container labelled as "poison" was found at Avalon Beach yesterday.

Another container was later found 300 metres north of the surf club at Palm Beach.

Fire and rescue hazardous material teams were called to secure the containers.

They are yet to test and identify the substances inside.

Superintendent Tom Cooper says there have been up to six containers with aluminium phosphide found along the coastline in recent weeks.

"We're not sure if these containers are related to a number of containers that have washed ashore over the past two weeks in New South Wales," he said.

"We've had a number of containers washed up - about five or six containers."

He says fire crews have no idea where the containers have come from, but one possibility is they fell from a ship during recent storms.

"We do believe that the recent cyclone storms in north Queensland have washed a lot of stuff on shore along the east coast of Australia," he said.

"We believe that's what's causing these containers to wash up."