The number of people infected by a salmonella outbreak in northern Tasmania grew to 28 yesterday when another eight cases were reported to health authorities.

One person is in a serious condition and two others have needed hospital treatment after eating products from two Launceston bakeries, which have voluntarily closed since the outbreak was discovered.

Owners of the Trevallyn Bakery and the Sugar Shack Bakery are working with the Public and Environmental Health Service to identify sources of the infection.