If you're pregnant and worried about mercury levels in New Zealand seafood, you don't necessarily have to steer clear of your favourite fish and chip dinner.

But be wary of trout - especially if it comes from geothermal regions such as North Island lakes (mercury is produced from volcanic emissions).

And some fish are best avoided or only eaten once a week or fortnight during pregnancy, including long-life fish and larger species such as school shark, swordfish, marlin and cardinal fish.

The advice from the Food Safety Authority has emerged after an Australian study found young children should avoid fish containing mercury.

New research published in the Medical Journal of Australia suggests that even at relatively low levels mercury can affect children's brain development.

Researchers examined three cases in Sydney where children aged between 15 months and 2 years had elevated mercury levels after eating up to five times the recommended dietary intake of fish.

The children had been fed congee - a rice and fish porridge popular in some Asian communities - and all displayed some behavioural problems such as aggression and autism and delayed speech development.

Jim Mann, professor in human nutrition at Otago University, said it was a concern that the children's blood levels showed high levels of mercury.

But he had doubts about whether that could be linked to the reported behavioural problems.

"That is is not a clinical trial, what are they comparing them to? It is very difficult to prove."

He doubted it was a concern for the New Zealand population. "Most people don't eat that much fish," he said.

The Food Safety Authority said fish and other types of seafood were highly nutritious, particularly for pregnant mothers, because omega-3 fatty acids were important for the baby's central nervous system development.

Fish was also low in saturated fat and an excellent source of protein, iodine and some vitamins.

"There are, however, unresolved issues about the levels of mercury in some seafood types and the potential impacts on the growing fetus."

At high levels of exposure, mercury could harm the nervous system, the authority said.

"The body over time excretes mercury so accumulation is also not a problem. However, a developing fetus is potentially more sensitive to the effects of mercury."

The council's website said there had been no cases of mercury poisoning from seafood reported in New Zealand where the average intake of mercury was less than a quarter of the recommended maximum safe level.