Key Points:

Health authorities launched an investigation today to identify the source of a nation-wide salmonella gastroenteritis outbreak.

The Ministry of Health, Institute of Environmental and Scientific Research (ESR) and public health services combined forces to conduct tests on multiple samples in hopes of locating the infection's source.

The investigation would be labour-intensive and challenging for all involved, ministry chief advisor on population health Greg Simmons said.

Since mid October 40 cases of salmonella gastroenteritis had been reported from Auckland to Southland, he said.

Those who were would infected suffered fever, headaches, abdominal pain, diarrhoea, nausea and vomiting.

Mr Simmons said for those suffering from salmonella gastroenteritis there were simple ways to prevent its transmission including avoiding the preparation of food and washing hands thoroughly.