The outbreak of the "unusual strain" of salmonella has now infected 26 people across NSW and the ACT according to NSW Health, leaving nine people in hospital and prompting the company to call in industrial cleaning experts.

Twenty four of the salmonellosis cases are at seven IRT centres in the ACT, Illawarra and South Eastern Sydney, with two other cases in centres administered by other NSW providers.

IRT Group chief executive Nieves Murray said the two Canberra cases were identified between January 18 and February 5.

Residents and their guardians were formally notified of the issue on February 6, according to the company.

Health authorities have also warned that it was possible that further cases may arise as investigations continue.

IRT owns a 100-room aged care home at their Kangara Waters property in Belconnen, as well as running in-home care services.

Ms Murray confirmed two of the Belconnen centre's elderly residents had contracted the disease on Tuesday afternoon, saying they were working with ACT Health to find the cause of the infection.

"The NSW Food Authority and ACT Health have conducted rigorous scientific testing at the affected care centres ... over the past two weeks," she said.

Ms Murray declined to give any details of the current condition of the two Belconnen patients, citing privacy concerns.

She said that the source of the interstate infections was still a mystery with the NSW Food Authority and ACT Health finding no evidence of salmonella in their kitchens.

Despite the finding, she said IRT has brought in outside specialists.

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

She also said they had reinforced food hygiene and infection control procedures.

The company first removed the "at risk" foods on Wednesday, February 4, and Canberra residents were advised of the Salmonella outbreak on Thursday, February 5.

Then, on February 6, residents and their guardians were advised of the issue in writing.

The provider confirmed that the second aged care home resident had died at one of IRT's Illawarra aged care facilities on Monday afternoon.

The first death occurred late last week and was confirmed by NSW Health Communicable Diseases Branch director Dr Vicky Sheppeard.

"To our best understanding, the salmonella infection was the cause of death of one elderly patient, in the Illawarra region, who also had several underlying chronic diseases.

"The second person at the Illawarra Shoalhaven aged care facility who had been affected by salmonellosis a couple of weeks ago had recovered from the illness, but subsequently died," Dr Sheppeard said.

Dr Sheppeard did not believe there was any risk to the wider community, and they were investigating food supply chains.

"Agencies are still working closely with a regional food supply company and the aged care facility managers at the affected facilities to identify foods associated with the illness," she said.

NSW Health said cases of salmonella first appeared in aged care homes on January 21.