Sixteen confirmed cases of salmonella - and at least eight suspected cases - have been identified at six residential facilities.

More than one aged care provider has been affected, however IRT has confirmed that 15 of the cases have occurred across its care centres.

Illawarra Shoalhaven Local Health District public health director Curtis Gregory said the confirmed cases occurred between January 21 and 29, although some of the suspected cases were more recent.

Mr Gregory said the public health unit was working with Health Protection NSW and the NSW Food Authority to identify the potential source of the infections and to minimise any risk to the community.

"Because the cases have occurred in the elderly population within care facilities, illness can be more serious than what you'd expect to see in the general population," Mr Gregory said.

"The elderly residents are mainly suffering from vomiting and diarrhoea but there are also issues around hydration and some have high fevers because of the bacterial infection.

"Three residents have had to be hospitalised while the remainder are under the care of their own GPs who will likely treat the symptoms and ensure they are hydrated."

Salmonellosis is one of the most notifiable conditions in NSW with more than 3000 people diagnosed each year and many more suffering illness but going undiagnosed.

The bacteria is mainly spread to people when they eat under-cooked food made from infected animals - including meat, poultry, eggs and their by-products - or salad items fertilised by manure.

Cross-contamination can occur when pre-prepared food is cut with a contaminated knife or via the hands of an infected food handler.

Mr Gregory said it was important to note that no staff members at the facilities had become ill, while inspections of the kitchens and food-handling procedures had not uncovered any issues.

"We are tracing the food provided to these patients ... we suspect there may be an issue with a supplier," he said.

"When we get a case of salmonella we take a serotype - or genetic marker - of the bacteria and we've done this on 12 of the confirmed cases so far and they all have the same serotype.

"It's a fairly uncommon type of salmonella called bovismorbificans and we think it is linked to salad products.

"Specifically we believe it may be due to the fertiliser or cow manure used to grow those salad products."

Mr Gregory said the most recent incidence of food poisoning due to this type of salmonella had occurred in 2011 in the United States and had been traced back to sesame seeds.

The most recent Australian outbreak of salmonella bovismorbificans was in Queensland in 2001, and that was eventually linked to shredded iceberg lettuce.

Mr Gregory said the aged care operators in question were alerting their residents and families, as well as taking measures to stop further infection.

IRT chief operating officer Craig Hamer said the organisation was working with the authorities to determine the infection's source.

"We are currently monitoring incidents of gastroenteritis-like symptoms at our care centres potentially associated with bacterial infection," he said.

"We are currently aware of 15 cases of salmonella across our care centres.

"As the safety of our residents is our highest priority, we have taken a number of precautionary measures to minimise risk across all our sites."

This included the temporary withdrawal of "at-risk foods" from service.

"As an extra precaution [we] have engaged external specialists to undertake industrial cleaning in our food service areas at affected sites," Mr Hamer said.

A NSW Food Authority spokesman said its staff were investigating the food storage and preparation facilities at all sites.

"This investigation involves environmental swabbing and sampling at various ... sites, audit of food eaten by residents and audit of food suppliers."

lwachsmuth@fairfaxmedia.com.au