A giant freshwater crayfish, weighing more than 3 kilograms, has been discovered in a rainforest in Tasmania's north-west.

The huge crustacean was found on the first day of the Tarkine BioBlitz, in which 160 participants surveyed sites in the contentious Tarkine forests, plains, rivers and coastline.

The crayfish is the largest freshwater invertebrate in the world, found in remote areas of the state's north-west, and conservationists want the area protected from logging.

Crayfish researcher Todd Walsh said it was the second-biggest freshwater crayfish he had ever caught.

"I was lucky enough to trap a massive male giant freshwater crayfish," he said.

"This big boy weighed over 3 kilos, with only one full-size claw, and with two full-size claws, he would have tipped the scales at close to 5 kilograms.

"This large male is the biggest I have trapped in almost 40 years."

The giant freshwater crayfish was tagged and released without harm.

One-clawed giant freshwater crayfish. ( Supplied: Mark Horstman )

Mr Walsh said the specimen was found in a rainforest unprotected from logging.

He said the location has been recommended for protection for over a decade, but nothing has been done.

"The site was low in sediment, with superb bush extending as far as the eye can see," he said.

"This huge creature is an example of how these animals will thrive in near-pristine environments."

For more than two decades, the giant freshwater crayfish has been listed as a priority species under the Regional Forest Agreement, according to Mr Walsh.

"We've had governments trumpet how important it is and yet not one of them has put a hectare aside specifically for this animal," he said.

"It is now well past time for Government to put selected areas aside to ensure these giant specimens are no longer such a rare occurrence."

Mr Walsh said if 30,000 hectares of rainforest was protected from logging and loss of riverside habitat it would "secure the future of this wonderful animal".

It is estimated that fewer than 100,000 remain in the wild, with numbers declining due to logging and land-clearing.

A spokesman from the Environment Department said the State Government was committed to the ongoing survival of the giant freshwater lobster in the wild, which is why it has made a recovery plan for the species jointly with the Australian Government.

"The habitat is currently protected under the Forest Practices System where streamside reserves are provided around known lobster habitat," the spokesman said.

He said the Government was working on a new recovery plan to better understand the species and its habitat.