People are being warned against eating uncooked semi-dried tomatoes after evidence emerged of a possible link with hepatitis A, NSW Health says.

An investigation into an outbreak of the disease in Victoria found 66 per cent of those who had fallen ill had eaten semi-dried tomatoes.

NSW Health's director of communicable diseases Jeremy McAnulty said public health investigators suspect the affected tomatoes are sold loosely in delicatessens and are likely to be served in restaurants, cafes and sandwich shops.

"Despite a comprehensive and complex investigation, no single supplier of semi-dried tomatoes has been proven to be to the cause of the Victorian outbreak," he said.

More than 80 people in Victoria have recently contracted the disease, while five cases have been detected in NSW since August 15.

It is believed three of those are related to the contaminated tomatoes.

"We are working closely with the Victorian health authorities and will continue to monitor the situation closely," Dr McAnulty said in a statement.

Hepatitis A is spread when traces of faecal matter containing the virus contaminates hands, objects, water or food and is then taken in by mouth, NSW Health said.

Symptoms include fever, nausea, poor appetite, abdominal discomfort, dark urine and jaundice.