Five years ago, the thought of Sydney growing a sub-tropical reef of its own would have caused David Booth to roll his eyes.

But now, after exploring the proliferation of Pocillopora aliciae off the coast of Manly, the professor of marine ecology has changed his tune.

"With what I've seen offshore, where seeing this large increase of the area of the sub-tropical corals, it suggests, yeah ... Sydney could have a reef in the future," he said.

According to Professor Booth, tropical fish began swimming south to Sydney two decades ago.

Initially, few species survived the cool winter water temperatures, but as Sydney's waterways get warmer, a number of tropical fish are beginning to call the city home.

Headband humbug fish mainly exist at the Great Barrier Reef, but here's one sheltering in coral off Manly. ( Supplied: John Sear )

"Since the 1920s there's been a change of 1.5 degrees Celsius in the winter water temperature in the Sydney region," the professor said.

"If the temperature gets above 18C in the winter and stays there, the fish tend to survive.

"We've seen more than 100 species arriving here, and probably five or six of them are the key ones that have managed to last to the next year."

The surviving species include sergeant major damselfish and the dusky butterflyfish.

David Booth and fellow UTS researcher Gigi Beretta snorkel at Shelly Beach to study the tropical fish. ( ABC Radio Sydney: Harriet Tatham )

Tropical fish and their new home

Coined as tropical vagrants by some experts, the survival of these species is helped by the growth of a sub-tropical coral species.

Pocillopora aliciae, known for its dark brown-green sprawling branches, was first detected off the Manly coast in 2015 by underwater photographer John Sear.

"I've been diving at that aquatic reserve for 25 years and I'd never seen it before," he said.

"It was so exciting to see that I let the university [UTS] know immediately."

Pocillopora aliciae is flourishing off the coast of Manly. ( Supplied: John Sear )

At first, the coral colonies Mr Sear discovered were no bigger than a football.

Five years later, there are sheets of coral measuring up to 25 metres — a change he finds both exciting and alarming.

"Initially [it was] very exciting to find corals in Sydney because we don't have many corals," he said.

"But I think after several years of studying it, I'm starting to wonder what the implications may be."

Underwater photographer John Sear found the coral in Sydney in 2015. ( ABC Radio Sydney: Harriet Tatham )

As far as Professor Booth is aware, this is the furthest south any tropical coral has been recorded.

"It's a true reef coral, and it's attracted to it a new suite of tropical species of fish we haven't seen here before," he said.

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A saving grace?

And while the arrival of these corals could be seen as a gradual change, Professor Booth said a situation in Japan suggested it was a sign of things to come.

"At 34 degrees north [of the equator] — and Sydney is about 34 degrees south — they've seen the proliferation of large sheets of branching coral to the extent that the kelp forest has disappeared," he said.

"In the past decade it's been a real shift from a kelp and rocky reef-dominated fish community to a coral-dominated community — and that happened very, very quickly, so it could happen here."

Last month, the long-term outlook for the Great Barrier Reef was downgraded from poor to very poor by the Federal Government's five-year reef report.

With this comes a question about whether the sub-tropical reef in Sydney could perhaps offer a measure of hope, a buffer to the bleak prediction for Australia's north.

Put simply, Professor Booth says no.