New South Wales Health and the state's Food Authority are investigating an outbreak of Hepatitis A, with 12 cases confirmed across Sydney and its surrounds over the past five weeks.
The numbers are considerably higher than the average of two cases of locally acquired hepatitis A each year.
NSW Health's director of communicable diseases Vicky Sheppeard said when outbreaks occurred in Australia, they were usually linked to the consumption of contaminated food products or person-to-person spread.
"So far there are no foods in common across these people nor are there common food outlets such as restaurants so we're still investigating to find any links between the cases," she said.
"It is possible if we don't find the source then others will continue to be infected.
"Our investigation doesn't find close links between all the 10 locally acquired cases and those who travelled overseas so we are concerned that rather than the source of this outbreak being from overseas, that there might be a food in NSW that has caused the infection of all these people."
The majority of the cases that have been confirmed are in Sydney's south-east, with some in northern Sydney, one in the Hunter and one in Wollongong.
In 2015, at least 18 people contracted the disease in Victoria with the outbreak linked to frozen berries.
There have been between 41 and 82 cases of hepatitis A notified to NSW Health each year since 2013, mostly in people returning from high-risk countries.
