A Northern Territory fish expert says if a team of scientists can find significant numbers of rare sharks, rays and swordfish, it could downgrade their critically endangered classification.

The team of scientists and conservation volunteers will join traditional owners in Arnhem Land until the end of June then move on to net the Roper, McArthur, Daly, Victoria and Keep rivers.

The Northern Territory Museum and Art Gallery's Dr Helen Larson says little is known about the rare creatures and it is hoped the team can take DNA samples and record known habitats and breeding behaviour.

"It's possible that the survey will show that their classification may be downgraded a little," Dr Larson said.

"We suspect the Northern Territory really has healthy populations of these things and they're just highly restricted.

"They need only very particular parts of the river to live in," she said.