Anyone bitten by a blue-ringed octopus can start to experience numbness and breathing difficulties within minutes. If left untreated, the bites can be fatal.

Nicola Powell was walking her dog at 6.30pm at Penguin's Watcombe Beach on Tasmania's north west coast when she saw a blue-ringed octopus.

"My 11-year-old stepson and I were down there walking our dog as we do pretty much every night. We were walking around the rockpools, and I always say watch out for blue-ringed octopus," she said.

She said it was a good opportunity to show her stepson what a blue-ringed octopus looks like.

A blue-ringed octopus is nocturnal and displays its iridescent blue markings when feeling threatened but usually looks sandy-coloured to blend in with the environment.

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Ulverstone Surf Life Saving Club patrol captain Murphy Knowles said, "anyone who saw a blue-ringed octopus, or any small octopus, should leave it alone."

"Please take care if you are in the vicinity and spot a blue-ringed octopus. You can avoid them by staying clear and not disturbing these environments," he said.

Ned Reardon from Surf Life Saving Tasmania encouraged people to get familiar with identifying a blue-ringed octopus and treatment of bites, and that people should always seek advice from patrolling members.

Mr Knowles said to always swim at a patrolled beach between the red and yellow flags, look for and obey the safety signs, and ask a lifesaver for help and advice.

If someone gets bitten, call 000 immediately.

Visit the Beachsafe website for more information.