An unidentified jellyfish which washed up on a Hobart beach could be the largest ever seen in Australia.

The 1.5m monster was photographed by a family walking on the beach last month.

Their find coincided with Government scientists working to classify the new species, which has reached plague proportions around southern Tasmania.

"It is new to science, it's not yet been classified," jellyfish expert Dr Lisa-ann Gershwin said yesterday.

"Then all of a sudden here comes this huge one and it's like, wow, this guy's a whopper.

"As Australian jellyfish go this would have to take the cake." Dr Gershwin said the white and pink gelatinous lump was from the same group as the world's largest species, found in the Arctic, which grows to 3m across.

It is not known why the creature is enjoying a bumper season, and growing to sizes larger than normal.

Theories range from warmer water to a different chemical composition in the ocean.

"The super abundances that we've been seeing last year and this year are really quite unusual and we don't know why," Dr Gershwin said.

"We don't know if it's something that has changed in the water, that this is the new normal, or if it's just a blip." It could also be a problem, with the species at least classified as one thing - a pest. Each female potentially produces millions of offspring and they eat masses of plankton, fish eggs and fish larvae.

"Whenever the oceans go a little bit wobbly the jellyfish seem to be the ones that come out the most victorious," Dr Gershwin said.

"Something is going on out there.

We don't know why and we don't actually know what effect it's having."

The creature's sting is not lethal but it's painful, as Dr Gershwin knows from personal experience.

"It feels like being stabbed by an ice-pick," she said.

"It's worth staying away from."

- AAP