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Professor Roughan said this January could break that mark.

University of Technology Sydney professor of marine ecology David Booth said several years ago corals bleached in the harbour when ocean temperatures reached 26 degrees, with eyes now turned to how the coral would react this time.

He said while it was interesting to see new species migrate, it came at the detriment of other species used to cooler environments.

Professor Booth added that while marine animals that thrived in cooler ecosystems could relocate as warmer waters prevailed, there was only so far they could travel.

“Fish can disperse further south, but they will run out of room — once they reach the end of Tasmania it’s a long way to Antarctica,” he said.

Ocean swimmer Lauren Tischendorf, who recently became the first woman to swim solo around Lord Howe Island, regularly swims around eastern suburbs beaches.

“I definitely noticed at the beginning of December and end of November the water felt warmer than normal,” she said. “You can tell early in the morning because it is still fresh outside, but the water is quite warm and balmy.

“While that warmer water is enjoyable to swim in - you can go further without getting cold, without needing extra nutrition and rehydrating - but at the same time, it is not as good for our environment. ”

Swimmers at Melbourne’s St Kilda beach experienced sea temperatures of 21.3 degrees on Tuesday, about 0.5 degrees warmer than the monthly average, while Williamstown’s ocean temperature was 21.6 degrees - marginally higher than its monthly average of 20.8 degrees. The warmest temperature ever recorded for Williamstown in January was 24 degrees.