A leading nephrologist from Prince of Wales Hospital, Bruce Pussell, said the number of deaths at the Penrith centre would be considered "unusually high" at the satellite centres linked to his hospital.

"Seriously ill dialysis patients are treated in hospitals where there is a ratio of one nurse to every three patients and not at satellite centres," Professor Pussell said.

"While I am not aware of the situation at the Penrith centre, I would say that in any hospital if you have a cluster of unusually high deaths among patients it would require investigation."

Patients at the Penrith Satellite Dialysis Centre have signed a letter obtained by Fairfax Media claiming at least three patients have died in the past three months in circumstances they claim could have been avoided.

The 15 patients who signed the letter said there are no doctors at the centre and an inadequate number of nurses. They also said many of the patients who died were so chronically ill that they should have been treated in hospital rather than at the centre.

Patients whose conditions deteriorated had to wait up to 20 minutes for an ambulance to transfer them to Nepean Hospital, the letter said.

There had also been six "close calls", the patients said.

The local health district has confirmed 13 of the centre's patients died in the past year, but said the deaths were related to complications from their illnesses rather than inadequate care.

A patient who signed the letter, Melody Sherriff, said while nurses were able to call a doctor for advice, it was not the same as having a doctor on the ward.

About four months ago Ms Sherriff was receiving dialysis at the centre when she suffered pain in her abdomen and lost about 300 millilitres of blood when she went to the toilet.

Nursing staff called the doctor, who did not come into the centre but said Ms Sherriff could either go home or to hospital.

"Thankfully, I chose the hospital," she said.

"The doctors at Nepean ordered a scan and found I was bleeding to death from an artery in my large intestine, requiring an emergency operation.

"Situations like this are why we need a doctor present in the ward. We shouldn't have all these people on the ward who can go at any minute because they're at risk of co-morbidities like stroke if qualified staff aren't there to treat them."

Satellite dialysis centres were set up to treat stable and ambulatory patients, she said, but many of the patients attending the Penrith centre were seriously ill and should be receiving dialysis in hospital. Most major hospitals have at least one satellite dialysis centre to which they are linked.

The Western Renal Service nurse manager for the Nepean Blue Mountains Local Health District, Christopher Gibb, agreed that patients receiving dialysis at satellite units should be medically stable.

He said the concerns raised in the letter were been taken seriously and the district was investigating.

"There have been no deaths related to the dialysis procedure, however 13 patients suffering from pre-existing illnesses have passed away as a result of their chronic illness over the past 12 months," he said.

The ration of one nurse to every five or six patients at the centre was consistent with the national average, he said.