EVEN old tennis balls can harbour deadly wildlife in Australia.

A lethal blue-ringed octopus gave a Perth family a scare when it emerged from within an old tennis ball the children were playing with.

Kristy Edgelow says her children were playing with an old split tennis ball they found in the river in North Fremantle for around 20 minutes before they realised something was inside.

Camera Icon Children played with the ball for about 20 minutes before they realised the tiny creature was inside. Credit: Supplied, supplied

“My older son noticed something moving inside,” she told PerthNow.

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

The River Guardians, a Swan River community organisation, warned the lethal octopus is a cryptic animal that likes to “hide in crevices,” so parents should be cautious of what their children pick up.

Although the octopus is best known as a ocean-dwelling marine animal, the River Guardians advise they may be as common in the river as in the sea.

The highly toxic creature is one of the most venomous marine animals in the world.

It causes breathing difficulties, numbness and paralysis with its bite for which there is no anti venom, making the risk of death high without immediate medical intervention.