Heidi Hollis was working at an RSL Club in Evans Head on the northern NSW coast back in February when she and several colleagues tucked into a lunch of Spanish mackerel.

What followed was months of pain as food poisoning-like symptoms initially sent her to hospital for several days and left her with persistent nerve damage.

“I was unable to walk until a week ago,” Ms Hollis said.

The source of the ailment, though, was much worse than a standard dodgy meal. Rather it was triggered by toxins contained in certain fish that are difficult to detect and can’t be destroyed by the cooking.

Known as ciguatera fish poisoning, the illness has been mostly tropical. Early Pacific explorers described it as far back as 1606 and even in 1776, during the second voyage of James Cook. More than 1500 cases have been documented in Australia since 1965 and the number appears to be rising.

The toxins are produced by marine microalgae, particularly the gambierdiscus genus. As fish graze on the algae and are then eaten by other fish, toxins accumulate up the food chain to levels harmful to humans.

This year, NSW had its first cases of the illness from fish caught in the state’s waters, with Food Authority NSW documenting four at Evans Head – including Ms Hollis – and Scotts Head, also in the state’s north.

Researchers, including Shauna Murray from the University of Technology, Sydney, have also detected gambierdiscus as far south as Merimbula for the first time.

“We never realised until now that they could survive in such cold conditions,” Dr Murray said.

She said it’s difficult to tell for certain whether the Merimbula microalgae  – which don’t contain the ciguatoxin – had been there all along or was a recent arrival. What is known, however, is that the East Australian Current is strengthening because of global warming, bringing warmer water and sub-tropical species into previously more temperate seas.