Tropical corals have been identified off Sydney's northern beaches and are "absolutely proliferating", providing habitat for a range of other species typically found much further north, according to the diver who found them.
Josh Sear, an underwater naturalist and photographer, has watched branching corals - whose scientific name is Pocillopora aliciae - gain purchase on large sandstone boulders in the Cabbage Tree Bay area near Manly in the past couple of years.
Initially in waters of about 13-15 metres depth, the corals have spread to the nearby popular Shelly Beach region and are found at depths of about three metres.
Remarkably, the corals are already providing habitat for a range of tropical fish and crab species not normally found this far south.
"Every year there are new ones coming down," Mr Sear said. "These are way out of their normal range."
Species are shifting globally as temperatures rise with climate change.
Corals in Japan, for instance, have been moving poleward at the rate of 14 kilometres a year. In Australia, it is the strengthening and warming East Australian Current driving species southwards along the east coast.
David Booth, a marine ecologist at the University of Technology Sydney, said the corals being found off Manly had previously been recorded off Port Stephens about 120 kilometres to the north.
"As far as we are aware, this is the furthest south of any tropical coral recorded," Professor Booth wrote in a paper published last month in the Coral Reefs journal.
"There's a thermal niche that didn't exist before," he told Fairfax Media.
Corals "are the start" for species migrating since they provide shelter and food for a range of other creatures such as fish and crabs that are now showing off Sydney, he said.
"It's like new apartment blocks have arrived in town," Professor Booth said.
A mystery about the multi-species migration, though, is "how the heck they got here", he said.
For instance, Mr Sear has identified and photographed a species of goby and shrimp that co-exist in the tropics normally 1000 kilometres to the north.
The shrimp is blind and digs holes for the goby, which in turn protects it.
Even with the symbiotic relationship, it remains unclear how both species so quickly arrived this far south, Mr Sears said.
Professor Booth said the key may be the increasing ability of coral larvae and other species to survive through winters as waters warm.
So far, there is little sign that the newcomers have dislodged other species in the Sydney areas at least, although it has happened elsewhere along the east coast.
"The big boulders of sandstone are just perfect for corals," Professor Booth said, noting the area is known locally as "the Barrens". They're "not so barren now", he said.
The Cabbage Tree Bay area is a protected zone, and earmarked to form part of the NSW government's proposed marine park for the Sydney region.
Tropical species may be expanding their range with warmer waters but the same force - climate change - is also playing havoc where sea temperatures rise too high.
A paper out on Wednesday in Nature Communications found the mass bleaching that killed about half of the Great Barrier Reef's corals over two consecutive summers did not spare deeper reefs either.
The researchers, including Ove Hoegh-Guldberg and other Australian-based scientists, found a summer upwelling of cooler water had provided "thermal relief" for corals in 2016.
That upwelling, however, subsided, leading to severe bleaching of 40 per cent of the deep reefs and about 6 per cent mortality, the paper found.
While much less than the death rates of shallow-reef corals, the delayed impacts of the bleaching may have triggered more mortality after the surveys were done.
The researchers noted deeper corals could provide a source of larvae that can help the recovery of harder-hit shallow reefs. However, that aid would be limited since the deeper reefs are home to a relatively small proportion of species compared with those closer to the surface.