Marine ecologists from the University of New South Wales say they have successfully restored a seaweed species that once thrived along Sydney's coastline.

The species, commonly known as crayweed, vanished along a large stretch of Sydney's coast during the 1970s and 80s, most likely due to high levels of sewage outfalls.

Crayweed is a habitat for crayfish and abalone.

UNSW researchers, along with the Sydney Institute of Marine Science and the NSW Department of Primary Industries have now transplanted some of the weed from Palm Beach and Cronulla to around Long Bay and Cape Banks.

Lead researcher Dr Alexandra Campbell says the study suggests about 70 kilometres of the seaweed disappeared.

"We're not 100 per cent sure what happened our best idea at the moment is that it was due to the really high volume, low treatment sewage outfall," she said.

"That would come pretty much directly onto the beaches until the 1980s and 1990s in Sydney and we think that this species is particularly sensitive to that kind of pollution."

Despite improved water quality around Sydney after the introduction of better infrastructure in the 1990s which pumped sewage into the deeper ocean, the 70 km gap has never been able to recover naturally.

Dr Campbell says the results have been better than expected.

"It's not kind of high tech stuff. We kind of attached it to some mats and drilled the mats back into the ocean floor and the adults that we moved from existing populations back into Sydney were kind of attached to those mats," she said.

"But the babies they've had since being put back into Sydney have now attached themselves onto the rocks which is a new and exciting thing for restoration in this type of transplant.

"Seaweeds are the 'trees' of the oceans, providing habitat structure, food and shelter for other marine organisms, such as crayfish and abalone."

UNSW Professor Peter Steinberg, who is the Director of the Sydney Institute of Marine Science, says the results could provide valuable insights for restoring similar macroalgae marine ecosystems in Australia and globally, but further research is still needed.

"This kind of restoration study has rarely been done in these seaweed-dominated habitats, but our results suggest that we may be able to assist in the recovery of underwater forests on Sydney's reefs, potentially enhancing biodiversity and recreational fishing opportunities along our coastline," he said.