Water in estuaries such as the Myall Lakes and Lake Macquarie is warming at twice the rate of the ocean, new research has shown.
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Scientists believe the findings could have major ramifications for the state's $100 million aquaculture industry, which includes Port Stephens and Myall Lakes oyster growers.
More than 6200 temperature observations were taken in 166 estuaries along the 1100-kilometre NSW coastline for the 12 year study.
On average, the estuary systems experienced a 2.16-degree temperature increase, about 0.2 degrees each year.
"This is evidence that climate change has arrived in Australia; it is not a projection based on modelling, but empirical data from more than a decade of investigation," Dr Elliot Scanes, a marine biologist at the University of Sydney, said.
The study was the first project of its kind to consider a diverse range of estuary types on such a large scale.
"This increase in temperature is an order of magnitude faster than predicted by global ocean and atmospheric models," Dr Scanes said.
The study, published on Monday in the journal Nature Communications, was the first project of its kind to consider a diverse range of estuary types on such a large scale.
According to the Australian Bureau of Meteorology, air and sea temperatures in Australia have increased by about 1 degree since 1910. And over the past decade, air temperatures have increased 1.5 degrees as compared to the 1961 to 1990 average.
In addition to the impacts on marine and bird life, the researchers have highlighted the potential impact on the millions of people who depend on rivers, lakes and lagoons for their livelihoods.
"Estuaries provide services of immense ecological and economic value. The rates of change observed in this study may also jeopardise the viability of coastal vegetation such as mangroves and saltmarsh in the coming decades and reduce their capacity to mitigate storm damage and sea-level rise," Professor Pauline Ross, who leads the research group in the University of Sydney's School of Life and Environmental Sciences, said.
Port Stephens' aquaculture industry, which is primarily made up of oyster growers, is worth more than $42 million annually.
About 100 people are employed across 45 oyster farming businesses in Port Stephens and Myall Lakes.
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Port Stephens Shellfish committee chairman and Karuah oyster farmer Mark Hunter said he was surprised by the research findings, but agreed warmer estuaries would adversely affect oyster crops.
"The water up the top of the estuary can get quite warm in summer but to date we haven't had a problem with oyster crops," he said.
"If the water is getting hotter over time then that is something we will need to address as an industry," he said.
