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.