Not since World War II have Sydneysiders had to worry about underwater invaders in their harbour.
But now marine conservationists are concerned about subtropical coral flocking to cooler Sydney waters with the potential to usurp local species.
University of Technology Sydney researcher Jen Matthews says global warming could mean trouble for the harbourside city's coral.
The waters around Sydney are becoming home to the new subtropical species as they flee south from the Great Barrier Reef in search of cooler waters, thanks to the rise in sea temperatures.
They are likely to thrive in Sydney by being better able to handle rising water temperatures compared with the natives.
"While we found these (arriving) corals possess the machinery to withstand large changes in temperature, those temperatures could kill the existing corals in Sydney," Dr Matthews said.
"This is incredibly important when we consider the future of Sydney's precious ecosystems.
"Coastal Sydney falls in the temperate climate zone, so the establishment of subtropical coral populations raises some interesting questions."
But there is debate among the team at the Future Reefs program about whether the new coral are harmless refugees or invading pests.
There could be benefits to the new corals as they increase the biodiversity of Sydney's marine environment, including providing more refuges for fish, crabs and other corals.
Australia's coral systems are being adversely impacted by climate change and the Great Barrier Reef on the east coast is suffering frequent bleaching events brought on by high temperatures.
Likewise on the west coast, conservationists found evidence of a similar bleaching event at Ningaloo Marine Park in April.