Public health authorities are concerned after two new cases of salmonella emerged at aged care facilities in the Illawarra this week, bringing the total number of patients to 31.
Last week authorities were hopeful the outbreak of the food-borne illness at 10 facilities in the Illawarra and Shoalhaven, south-eastern Sydney and the ACT had been contained.
Two elderly residents have died and 16 residents have been hospitalised after contracting a rare strain of salmonella, with the onset of the first case on January 21.
Illawarra Shoalhaven Local Health District public health director Curtis Gregory confirmed that a new case was identified late on Monday, and another on Tuesday morning. He said the onset of the last case was now believed to be February 23.
"It is concerning as we'd hoped that it had been contained," he said. "What happens next will depend on whether we are seeing a second wave or just a couple of isolated cases at the end of an outbreak."
IRT said that 28 of the confirmed cases occurred at its care centres, while the other three cases were at centres supplied by IRT catering.
"We have put a range of measures in place to minimise risk to our residents," IRT chief executive Nieves Murray said.
"The last of these measures was put in place on the evening of February 17. The recent confirmed cases of salmonella are well within the incubation period for the infection, as advised by NSW Health."
Specialist industrial cleaning took place at all the affected sites last month while high-risk foods have been taken off the menu.
There have been no confirmed cases of salmonella among IRT staff.
Mr Gregory said the public health unit was collating the food histories of patients and other information as it worked with the NSW Food Authority to find the source of the outbreak.