Danger lurks in every corner of Australia and tennis balls appear to be no exception.

A Western Australia mum has told how her children were playing with a tennis ball when a deadly blue ringed octopus emerged from inside it.

Kristy Edgelow said her children had found the split tennis ball in a river in North Fremantle and spent about 20 minutes throwing it around before her son noticed something moving inside, Perth Now reports.

"We got the fright of our lives when this little fellow popped his leg out and we realised it was a blue-ringed octopus," she said.

The blue ringed octopus is among the most lethal marine creatures in the world.

There is no anti-venom for their poison, which causes breathing difficulties, paralysis and can lead to death without immediate medical treatment.