"Sydney will have tropical waters by between 2040-60," Adriana Verges, a marine ecologist at the University of NSW, said. "Summers [will be] above 25, winter 19 degrees."

Those celebrating the future demise of the wetsuit, though, might want to take a look beyond the shallows.

A paper published in Geophysical Research Letters this month highlighted the extent of warmth wasn't being captured by the readily available surface temperature measurements.

"Satellites are not getting the full picture," Moninya Roughan, an associate professor at UNSW's Coastal and Regional Oceanography Lab and co-author of the paper said. "They are missing the peak and intensity, and sometimes the duration [of marine heat waves]."

Professor Roughan and her colleague Amandine Schaeffer used data from two offshore moorings to create one of the first long-term assessments of temperatures from the surface to the seabed as deep as 100 metres.

Marine heatwaves off Sydney - based on at least five consecutive days when temperatures were in the top 10 per cent of readings - were found to last as long as a month. The average duration was between eight to 12 days.

The biggest average anomalies were at 50-metre depths and the most extreme temperatures were as much as 6 degrees above the norm, based on two data sets covering seven and 25 years.

"Weeks [of heatwaves] are a long time when you're a marine organism, a small creature, at the bottom of the food chain," Professor Roughan said.

While the data periods were too short to identify longer term climate trends, an abundance of research suggested marine hot spots were likely to get hotter and there would be worsening impacts.

The Great Barrier Reef was an area of much-publicised concern, where two warm summers in a row had triggered unprecedented coral bleaching; about two-thirds of the reef was affected.

But the East Australian Current skirting the eastern seaboard including Victoria and Tasmania was also changing, extending southwards about 350 kilometres in 60 years.

"The predictions and the models all indicate that [the EAC] will only continue to intensify," Dr Verges said.

An impact of the so-called tropicalisation of temperate waters was that herbivorous species, such as rabbitfish, silver drummers and sea urchins, were moving into rich ecosystems such as kelp forests.

"They are essentially destroying the habitat that is the foundation for the entire ecological community," Dr Verges said.

Bursts of heat can also take their toll. The extreme event off Western Australia in the 2010-11 summer - the worst recorded in 160 years of records - killed about 150 kilometres of the kelp's range, which had not recovered, Dr Verges said.

"We should be as concerned about the loss of kelp as we are for the loss of corals," she said, adding the 8000-kilometre stretch of kelp forests in Australia's southern coastal waters, with its abalone, lobster and other industries, generated $10 billion a year in economic activity.

Knowing more about the extent of heatwaves -and how they are changing - will be important to coastal communities everywhere, Professor Roughan said.

"They are the most productive regions on earth ??? and so maximum ecosystems damage will also occur," she said.

Scientists said the research benefited from having long-term uninterrupted data sets supported by the National Collaborative Research Infrastructure Strategy and the Integrated Marine Observing System, the type of programs the CSIRO sought to cut last year.