Ten of the 12 hepatitis A patients contracted the disease in Australia, considerably higher than the average two locally acquired cases per year, said Dr Vicky Sheppeard, Director Communicable Diseases with NSW Health.

"NSW Health is working with the NSW Food Authority to investigate the outbreak, including assessment of patterns of food distribution and any links to overseas outbreaks," said Dr Sheppeard.

No specific food has yet been connected to the outbreak, she said.

All 12 infected adults are recovering. Three patients needed to be hospitalised.

Two patients are from the same family, but otherwise there are no known links between the cases, Dr Sheppeard said.

Hepatitis A is usually contracted overseas in high-risk countries, but 10 of these 12 people notified to NSW Health since July 26 have had no recent overseas travel, Dr Sheppeard said.

"Travellers to high-risk countries and anyone at higher risk of infection, including men who have sex with men, injecting drug users, sewerage workers and childcare workers, should ensure that they are vaccinated against hepatitis A.

"Two doses of vaccine prevent infection and is available through GPs," she said.

Hepatitis A is caused by a virus that spreads in contaminated food or through poor hygiene. Symptoms may include nausea, vomiting, fever and yellowing of the skin, dark urine and pale stools.

The disease can cause infections and damage the liver.

Globally, Hepatitis A is the most frequent cause of food-borne infection, but Australia has very few hepatitis A cases. Since 2014, NSW has had between 41 and 82 cases of Hepatitis A each, mostly in people returning from high-risk countries.

When outbreaks do occur they are linked to contaminated food. The disease is then spread person-to-person.

In 2015, at least 18 people contracted the infection in Victoria during an outbreak linked to Patties Food Nanna's brand frozen berries.

Dr Sheppeard said none of the 12 patients reported eating frozen berries and molecular sequencing indicated that the cases were not related to the earlier frozen berry outbreak.

Several hepatitis A outbreaks have been reported internationally in the past six months where hepatitis A is usually uncommon, including in Europe and California.

Washing hands thoroughly, particularly after going to the toilet, touching soiled linen or items, changing nappies, and before preparing or eating food can prevent the spread of Hepatitis A.

The Hepatitis A vaccine is on the National Immunisation Program Schedule and funded for all Aboriginal and Torres Strait Islander children aged 12-24 months living in Queensland, Northern Territory, Western Australia and South Australia.