The common coral trout usually calls the warmer waters of Queensland home.

But on the weekend, local spearfisherman Derrick Cruz found a mature, 2.8 kilogram orange specimen a little north of the Cape Banks region in the city's eastern suburbs.

Mr Cruz, a doctoral student researching native fish at UNSW, was taken aback. His was the southernmost sighting of the common coral trout in Australia.

The UNSW ecologist Adriana Verges described the find as "truly remarkable" but pointed to a "clear increase in the number of warm-water species in cool latitudes."

Butterfly, surgeon and parrot fish have all been found in Sydney waters.

Dr Verges said the shifting ecosystems were caused by ocean warming and the intensification of the east coast current.

"Tropical species coming down south is definitely a reason to be alarmed," she said.

"The waters off Sydney will have typical tropical temperatures like those found off the southern Great Barrier Reef in about 40 to 50 years."

Dr Verges said tropicalisation could pose a threat to Australia's commercial fishing operations; warmer temperatures had already hurt Japan's abalone and crayfish populations.

Mr Cruz has donated his find to the Australian Museum's fish collection for research but not without some reservations.

"Coral trout is a very tasty species," he said.

"But as much as I would have liked to have eaten it, I thought guys at the museum would appreciate it a lot more than me."