Such a jellyfish has been seen in the past, but not one one so large and not one that became beached, said Lisa Gershwin, a scientist with the government's Commonwealth Scientific and Industrial Research Organisation (CSIRO).
"We know about this specimen but it hasn't been classified yet, it hasn't been named," said the scientist who has been working with jellyfish for 20 years.
"It is so big it took our breath away.
"It's a whopper of an animal but it's not life-threatening, although it does sting."
The unclassified species is related to the lion's mane jellyfish, the largest known species of the marine animal in the world.
Ms Gershwin said there had been a huge jellyfish bloom in Tasmanian waters over the past month.
CSIRO scientists now have enough pictures and samples to begin a proper analysis and give the creature a name.
Despite this, much remains unknown, including how it eats and breeds, and its habitat.
"It's so big but we know nothing about it," said Ms Gershwin. "It highlights again how much we still have to learn about the ocean."
The jellyfish was found by the Lim family on a beach south of the Tasmanian capital Hobart .
Mother Josie said: "It blew our minds away.
"It's not really jellyfish territory here and all we could do was stand back and admire it."