Sydney scientists are using the marine equivalent of facial recognition software to photograph and track weedy seadragons off the coast of New South Wales, Victoria and Tasmania.

They fear the species, which is endemic to Australia, could be dying out in some areas and may need to be reclassified from near threatened to endangered.

Selma Klanten, a marine biologist at the University of Technology Sydney (UTS), is conducting a two-year study into the species, which is found in waters from Port Stephens, north of Sydney, across the south of the continent including Tasmania and in Western Australia.

"They are iconic and they're endemic. Once they're gone, they're gone. And once we lose them, we will never get them back," Dr Klanten said.

Dr Klanten has enlisted the help of expert divers and citizen scientists, including Kris O'Keefe. These divers photograph the fish and analyse their colourful flanks.

"The software program allows us to map out the markings on the side of a weedy seadragon and it becomes a unique fingerprint for the weedies," Ms O'Keefe said.

"And it will tell us if it's David or Greg or Alisha" Dr Klanten added, "Because every single new seadragon we have gets a name so we can follow it over the years."

"You don't have to handle it, you don't have to put tags on it. It's just a beautiful photograph that will help us."

Researcher John Turnbull said the kelp that the seadragons depended on appeared to be thinning out at Kurnell in Sydney's south.

"The numbers here are down. This is the best site in Sydney, so you can still see them here reliably," he said.

The technology will allow scientists to track weedy seadragons without having to tag them. ( Supplied: Kris O'Keefe )

Seadragons now further out of Sydney Harbour

'Dragon alley', a dive site off Barrenjoey Head at Sydney's northern most point, is no longer home to the species it was named after.

Scientists say that Botany Bay, where Captain Cook landed nearly 250 years ago, has also been devastated.

UTS professor of marine ecology, David Booth, said there were no seadragons there now at all.

"The outer sites are doing fine at the moment, but clearly industrialisation and sedimentation has reduced the kelps which in turn has meant the seadragons are probably not dead, but have relocated further out of the harbour," he said.

Rising ocean temperatures were a concern too he said, warning that research on the Great Barrier Reef showed fish stocks could quickly disappear.

"With climate change driving warmer waters further south, I think the Sydney populations could come under threat directly through climate change," he said.

"All of a sudden a threshold is reached and a species is gone. So even though we think there's going to be a steady decline and we can monitor it, it can happen suddenly and that's the fear," Professor Booth said.

This baby weedy seadragon measures about 10 centimetres. ( Supplied: Kris O'Keefe )

Scientists 'shocked' to find no seadragons in River Derwent

Dr Klanten said a 2016 survey in Tasmania failed to identify any weedy seadragons in the River Derwent.

"We went to those three sites, did our surveys and did not find one single individual, which shocked us a little bit," she said.

"Because we needed some samples from Tasmania, we went up to Bicheno. There it's a very clear and beautiful area and we found eight."

Dr Klanten said locals were also worried about the seadragon population in the Melbourne suburb of Portsea.

"Since the channel got deepened in 2009 for the port development in Melbourne, wave action [has] destroyed the habitat," Dr Klanten said.

She said instead of seeing between 50 and 80 seadragons under the pier, they found only eight.

Each population has a slightly different gene pool and so far Dr Klanten has identified 45 individual species' DNA from NSW alone.

Because there is no genetic diversity, the fish are particularly susceptible to environmental changes.

"They were here hundreds of thousands of years ago. With our impact, losing them would be really sad," Dr Klanten said.

The first results from Dr Klanten's study, which is funded by Sea World on the Gold Coast, should be available in October.