Authorities are examining how berries contaminated with hepatitis A ended up being imported into Australia.

Prime Minister Tony Abbott has assured Australians all berry products involved in a hepatitis A outbreak are being screened as the threat of contamination spreads to more schools.

But it's unclear who, if anyone, is carrying out the screening.

The number of confirmed cases of hepatitis A linked to the imported frozen berries remained at 13 on Thursday.

But children at up to 20 South Australian schools and childcare centres may have eaten the contaminated berries.

And a primary school in Cairns has warned parents their children may have been served smoothies at the tuckshop containing the now-recalled berries.

Mr Abbott told reporters on Thursday 100 per cent of "these sorts of imports" are now screened, despite not yet being classified as high risk by the food standards regulatory body FSANZ.

"We have moved to 100 per cent screening of these sorts of imports until this matter is resolved in a way which is very protective of the health of the Australian public," Mr Abbott said.

The prime minister's office later clarified Mr Abbott was referring to the Patties Food products that were involved in the outbreak.

However, a spokesman couldn't confirm who was doing the testing.

The Department of Agriculture wouldn't confirm any changes in screening but a spokesman told AAP that parent company Patties Foods was voluntarily holding all products involved.

The berries at the centre of the outbreak, branded in Australia as Nanna's and Creative Gourmet, have been recalled and taken off shelves.

Frozen berries are classified low risk, requiring only five per cent screening, but the department has requested a review of that status.

Patties Foods chief executive Steven Chaur told the ABC tests at the farm and production facilities for various diseases and bacteria had come up negative.

Opposition Leader Bill Shorten said while the viral outbreak had shaken confidence, it could encourage more people to buy Australian-grown produce.

He's demanding the government explain fully how it's making sure imported food won't make people sick.

Consumer watchdog boss Rod Sims said it was great for consumers if companies could import cheap goods but they had a responsibility to make sure the imports were safe.

"The less expensive the thing you buy, the more you need to check that you've got the processes in place to make sure that it is safe," the Australian Competition and Consumer Commission chairman told a business lunch.

Health authorities around the country say the risk of illness is low but they are taking a cautious approach.

The number of infections could rise over the coming weeks, with the Department of Health saying up to one in 100 people who ate the berries could contract the virus.