Scientists have found a selectively bred group of Sydney rock oysters are toughening up to protect themselves against the effects of climate change.

Key points: Increasing carbon dioxide emissions have driven acidity levels in oceans, damaging oyster shells and causing them to grow smaller

Increasing carbon dioxide emissions have driven acidity levels in oceans, damaging oyster shells and causing them to grow smaller Scientists said these new selectively bred oysters were designed to be disease-resistant and the new discovery was unexpected

Scientists said these new selectively bred oysters were designed to be disease-resistant and the new discovery was unexpected Sydney rock oysters are one of three species found in NSW, all of which are still threatened by ocean acidification

The accidental discovery was made by a team at the University of Sydney and Scotland's University of Stirling studying oysters on NSW's mid-north coast.

Oyster farmers in Port Stephens and Lake Wallace have suffered from poor harvests brought about by greenhouse gas-driven ocean acidity, leading to damaged shells and smaller oysters.

"What we've found is these selectively bred oysters are changing the way they make their shells," said Sydney University Biology Professor Maria Byrne.

"The more time an oyster spends fighting acidic seawater, the less time it spends growing.

"If they can make their shell hard, with less energy, they can make a bigger oyster. And that is better for the farmers."

Acidic seawater is caused by oceans absorbing higher amounts of carbon dioxide from the atmosphere.

NSW oyster farmers face several challenges, including acidic seawater. ( ABC News: Selby Stewart )

It erodes oyster shells, and has drastically reduced the size of Sydney's coastal rock oysters.

"Our farmers have huge financial investment in their farms and it's a massive output of work to maintain them," Professor Byrne said.

"Now we've found an oyster that is resilient to at least some of the challenges posed by a changing climate.

"If farmers know they can depend on these resistant strains, then that is massive for their business's longevity."

Sydney's rock oysters underwent a State Government-sponsored breeding program more than 25 years ago.

They were bred to be disease-resistant, and researchers said this new discovery was unexpected.

Oyster farmer Bruce Alford said those breeding programs have saved the $35-million-dollar industry.

"We are only in this business because of them, we would have had to close otherwise due to parasites and disease," he said.

"Up and down the coast there are more people investing in the industry and discoveries give people hope."

Oyster farmer Bruce Alford says breeding programs have saved the industry. ( ABC News: Selby Stewart )

Sydney rock oysters are one of three species found in NSW alongside flat and Pacific oysters, which are still threatened by ocean acidification.

The resistant oysters will now be tested in other water conditions to determine their suitability for global restoration programs.

"Now we need to see how the oysters go against other threats like freshwater run-off and pollution," Professor Byrne said.

"Then we can hopefully use them internationally to restore oyster reefs and farms that have been gradually destroyed by increasing acidity levels.

"It's a really exciting discovery for all of us."