* Thirteen Australians have hepatitis A, more cases expected
* Farmers, consumer groups push for changes to product labelling
* Tests continue on berries packed in China
By Jane Wardell
SYDNEY, Feb 20 (Reuters) - An outbreak of hepatitis A in Australia, probably caused by frozen berries packaged in China, is giving added impetus to moves to tighten the country's murky food labelling laws and could fuel a backlash against imported food.
Proposed changes that would more clearly identify the origin of food on supermarket shelves, combined with growing pressure on consumers to buy local produce, may curb the appetite for Chinese imports and could undercut a landmark free trade deal.
"I want to make sure I do everything in my power to say to people 'your safest food is your domestic food'," Federal Agriculture Minister Barnaby Joyce said on Wednesday.
Thirteen Australians have been diagnosed with hepatitis A, a viral disease affecting the liver, after eating frozen mixed berries sold by Patties Foods Ltd and health officials expect to identify more cases in coming weeks.
The berries were grown in Chile and China before being packaged at a Chinese factory, where poor hygiene and tainted water supplies are thought to have caused the health problems. Hepatitis A is passed through contact with material that has been contaminated with faeces from an infected person.
The case has given impetus to a bill introduced into the Australian parliament a week ago by the minority opposition Greens Party, which wants more informative labelling on products for sale in Australia.
Under current laws, the term "made from Australian and imported ingredients" is common, providing no detail on the exact origins of all the ingredients in a product or where it was packaged.
Imported food products have to undergo less stringent safety tests during and after production than local produce, farmers' groups say.
Joyce said the weak country-of-origin labelling laws allowed manufacturers to use "sneaky terms" to "earn a premium" on cheaply made products.
Farming and consumer groups say the case highlights long-term food safety problems in China, the third-largest source of Australia's food imports, where rapid industrialisation has polluted soil and water supplies. Poor hygiene in production and packaging plants is also a big concern.
"The current issue with imported frozen berries highlights the need for clearer country-of-origin labelling, as it appears consumers may have been confused about where they came from," said Ian Harrison, chief executive of lobby group Australian Made Campaign.
"Buy local" campaigns are increasingly prevalent in Australia and are gaining traction.
A July 2013 Roy Morgan poll showed more than 60 percent of Australians sometimes bought products because they were made in Australia even when they were more expensive than imported rivals. Only 12 percent said price would deter them from buying Australian products.
Prime Minister Tony Abbott, who late last year signed a free trade deal with China, is less enamoured with changing country-of-origin labelling laws, saying he is wary of imposing new regulations that could send food prices soaring.
Trade agreements make such changes tricky politically. Australia needs the trade pact with China to help it move from a reliance on exports of coal and iron ore to expanding its food and agricultural shipments to a growing Asian middle class.
There is also the risk of appearing contradictory. Australia is one of a number of countries that have taken action in the World Trade Organization (WTO) against the United States after it introduced its own stronger country-of-origin labelling laws for beef products last year.
Abbott said the government would look at toughening up screening processes for imported goods but put the impetus on the food industry to make its own checks.
Patties Foods has withdrawn the frozen berries from sale and stressed it carried out all the tests required under Australian law but it will probably face a class action lawsuit. Its shares have fallen 10 percent since it announced the product recall over the weekend. (Editing by Alan Raybould)