The Beddomeia tumida, a freshwater snail measuring around 4mm, had not been seen or recorded for more than 120 years.

Found only in Tasmania's yingina /Great Lake, researchers feared the minuscule mollusc was lost, before the exciting find in the Central Plateau lake in January.

Scientists weren't even looking for the pint-sized snail when it was found feasting on algae on a submerged tile.

Environmental scientists had been searching for the giant freshwater limpet - another snail species also believed to be extinct.

The giant limpet was rediscovered during a lake survey in 2015 and 2016 before scientists returned last year.

The team lowered roof tiles sourced from a tip shop to various depths to lure the limpet.

Within 10 days they not only found the limpet, but the elusive snail as well.

"This species of freshwater snail was thought likely to be extinct," Field Ecologist and Water Quality Consultant Kevin Macfarlane said.

"But subsequent surveys have recorded further specimens of both snails on other tiles, so far in relatively low numbers," he said.

Identification of the 4mm snail species was confirmed by senior zoologist and aquatic mollusc expert Dr Karen Richards and so far 15 have been found in total.

"It's a significant find because we now know it still exists," Mr Macfarlane said.

"Once we get an idea of population size, it could potentially have its listing changed to be no longer critically endangered, and hopefully we find that it's abundant."

The discovery is also a positive sign of health for the yingina/Great Lake system.

"As Beddomeia species are not present at locations of reduced water quality, this tells us that the water quality is very good, and the habitat is very good.

We've discovered not only undescribed freshwater snails in Great Lake, but numerous small crustaceans, and an aquatic plant called a charophyte."

Tasmania's Great Lake is the second-largest natural lake in Australia and was the first source of water for Hydro Tasmania.

Australian Associated Press