The unknown creature has experts working to solve the case, while swimmers are warned of possible dangers.
Launceston biologist Dr Lisa-ann Gershwin said the find was an incredible find.
“These thick tentacles scare me because the species with the thick tentacles are the dangerous species,” she said.
“(They are) the deadly species.”
Cronulla swimmer Scott Belcher came across the unknown jellyfish about half a mile from shore last week.
Belcher swims most mornings in Cronulla with the Shark Island Psycho Swimmers swim group.
“I’m the guy with the camera who was in just the right place at the right time,” he said.
“That might be the deadliest thing we have in Cronulla, I hope not.”
However, Gershwin said they couldn’t begin to understand how dangerous and poisonous it was until they learned what the exact species was.
Gershwin is now working with the Australian Museum to identify jellyfish.