By the Bureau of Meteorology's definition, a marine heatwave occurs when temperatures in the top 10 per cent of readings for that time of year exist for at least five days. The Tasman Sea has met the definition for months on end, two years in a row. Loading "A marine heatwave began affecting the Tasman Sea in December, with sea surface temperatures 2–3 degrees above average," said Andrew Marshall, a senior bureau research scientist. Last year's event was eve more extreme with some areas 4 degrees warmer than normal for extended periods. The effect on marine ecosystems, fishing and aquaculture industries "can be devastating, including killing off kelp forests and corals", he said. When cold waters turn warm

Earlier in the summer beach-goers along eastern Australia had the opposite experience, with prevailing winds producing an up-welling of cool water that made for some chilly dips for many holidaymakers. More recently other coastal influences have brought the warm East Australian Current closer to land, sending seawater temperatures back up to mid-20s. The conditions have contributed to an outbreak of marine algal blooms. WaterNSW on Saturday warned swimmers from Palm Beach in the north to the Illawarra in the south to avoid contact with brownish water "often mistaken as an oil slick". While not considered toxic, the algae can cause skin and eye irritation.

Madeleine Cahill, a physical oceanographer at the CSIRO, said the early-season cool upwelling brought nutrients to the surface that fed the algae. 'Not very happy' This summer's Tasman Sea heatwave was closer to Australia, compared with last year when it hit the waters around New Zealand and Kiwis had their hottest summer on record. (The summer just ended was likely the third-warmest on record, New Zealand's NIWA weather service says.) This year's unusual marine warmth might damage coastal fisheries in Australia. "Salmon, mussels, oysters are not going to be very happy,"Dr Cahill said.

Moninya Roughan, a professor at the University of NSW specialising in ocean circulation around Australia, said last year's more extreme event meant New Zealand's fisheries suffered, including a voluntary cut in quotas at the Hoki deepwater fishery that affected supplies to Australian markets. The heat isn't over either. Tasmania smashed its March maximum temperature records on Saturday with 39.7 degrees registered on Bruny Island. Hobart's top of 39.1 beat its old record from 2008 by almost two degrees. Loading The Tasman heatwaves are separate from the warming impacts caused by the strengthening of the East Australian Current, which brings tropical waters further south. However, climate change is understood to be playing an important role in both trends, scientists said. "Oceans absorb up to 90 per cent of the additional energy arising from the enhanced greenhouse effect," Dr Marshall said.