A previously unidentified species of giant jellyfish is invading southern Tasmania this summer, baffling scientists after one of the animals washed up on the beach, the Sydney Morning Herald reported.

One monster measured 1.5-metres.

"In Tasmania, we don't do jellyfish. This was something else. We've just never seen anything like it," said Josie Lim, who made the find.

Little did the Lim family know that, over at the CSIRO, Dr Lisa-ann Gershwin, who has been working with jellyfish for 20 years, had been hearing stories of this elusive animal in waters off Tasmania for more than a decade.

"The thing that I first said when I saw it [the photograph] was 'Phwoar'. It's a very scientific term," Dr Gershwin said.

"I'm just rapt by it, honestly. It's such an amazing find."

Dr Gershwin said curious people had been asking her for years about a big, white-ish jellyfish "with pink in the middle" in waters off Tasmania.

The jellyfish was said to grow quite large, and wasn't spotted very often.

In the years since, Dr Gershwin managed to obtain samples of two of the three species of jellyfish, which previously were unknown to science.

The third proved to be more difficult to track down - until this summer.

"All of a sudden I started getting all these calls, and all these people sending me photographs. Sure enough this thing is an absolute menace this season; it's been around in large numbers," she said.