Blue-ringed octopus have been spotted at beaches across Tasmania this month, but a lack of data on the deadly creature means it's unknown whether their numbers are swelling.

Two of the octopus were spotted at Devonport over the weekend and another in Wynyard.

Sightings have also been reported at Bellerive Bluff and Margate in the state's south.

The octopus is among the most venomous marine animals in the world, whose bite can cause total body paralysis and death if not treated.

Professor Gretta Pecl from the Institute of Marine and Antarctic Studies said blue-ringed octopus sightings in Tasmania were "uncommon but not unexpected".

"Blue-ringed octopus belong in Tasmania. They've been recorded here for quite some time."

If someone is bitten: Perform first-aid action plan DRSABCD

Perform first-aid action plan DRSABCD Calm the patient and keep them still

Calm the patient and keep them still Apply a crepe bandage over the bite

Apply a crepe bandage over the bite Firmly apply a heavy crepe pressure bandage, starting at the fingers/toes and working up as far as possible

Firmly apply a heavy crepe pressure bandage, starting at the fingers/toes and working up as far as possible Immobilise the bandaged limb using splints

Immobilise the bandaged limb using splints Ensure an ambulance has been called Source: St John Ambulance

Numbers growing worldwide

Professor Pecl said there was no data on the species' presence in Tasmanian waters, though octopus numbers globally were on the rise.

"We can't say anything about the abundance of them and whether or not there's more because there's no surveys that look at the number of octopus over time," she said.

"We need more information on the distribution of the species we have so we can say when they are changing distribution and look at factors like climate change or habitat loss."

Dr Pecl said anyone who saw a blue-ringed octopus, or any small octopus, should leave it alone.

"People should not pick them up, because any small octopus won't look like a blue-ringed octopus until it's about to inject venom."

The octopus is a master of disguise and only bites if provoked. ( Museum Victoria: Julian Finn )

'Extremely dangerous'

While some blue-ringed octopus measure only five centimetres, some have been found up to 20 centimetres in size.

Seahorse World aquarium manager Chris Carey said the creature was "extremely dangerous".

"They have a mixture of bacteria on their beaks that can shut down the central nervous system," he said.

The reported sightings from Margate to Wynyard did not come as a surprise to Mr Carey.

"They generally live in quite complex habitats like reefs or intertidal zones like rockpools," he said.

Blue rings are warning signs

Mr Carey the octopus were shy creatures and masters of camouflage.

"The only time people spot them is when they make themselves known and show those bright blue rings," he said.

"That's when they are warning you to stay away from them."

The octopus have short life cycles, he said, lasting only one to two years.

It caused two deaths in Australia in the 1950s and 1960s but are not aggressive and only bite if provoked.