More deaths of people with swine flu are inevitable but the growing list of fatalities should not trigger alarm, health authorities say.

A 26-year-old Perth woman late on Friday became the fifth Australian and second West Australian with swine flu to die.

The woman, who authorities say had a pre-existing serious medical condition, had been in Royal Perth Hospital's intensive care unit for several days.

A hospital spokeswoman said it was not known how or when she contracted the human swine flu virus.

She said the woman's family had requested that she not be identified and that no medical details of her condition be released.

A 26-year-old man from the remote WA Aboriginal community of Kiwirrkurra, west of Alice Springs, became the first Australian swine flu fatality when he died in Royal Adelaide Hospital on June 19.

A 71-year-old Melbourne woman who died on Thursday was the fourth Australian with swine flu to die, after a 50-year-old Victorian long-term cancer sufferer died in Melbourne on Wednesday and 35-year-old diabetic man died at Colac in western Victoria on June 20.

Authorities say all five patients had underlying medical conditions and none of the deaths could be attributed directly to swine flu.

But NSW authorities said a five-year-old girl being treated in intensive care at Sydney's Children's Hospital had no pre-existing medical risk factors for severe influenza.

She was admitted on Thursday and her condition was described as stable on Saturday.

Two nurses in two of Adelaide's major hospitals have tested positive for swine flu.

South Australia's chief medical officer Professor Paddy Phillips said both hospitals - the Royal Adelaide and Lyell McEwin - had undertaken infection control procedures to limit the spread and risk to patients, visitors and staff.

The Lyell McEwin Hospital nurse worked last Tuesday in a general ward but had not been at work since. She was confirmed as infected with H1N1 influenza late on Friday.

Patients she had been in close contact with have been treated with Tamiflu, with one still to be contacted.

The Royal Adelaide nurse worked on Thursday, became unwell and was confirmed as a positive case on Saturday.

"One patient was in prolonged contact with her and has been given Tamiflu as a precaution," Professor Phillips said in a statement on Sunday.

He said the number of confirmed cases in SA currently stood at 262 and was continuing to rise.

"But it is important to remember that in the great majority of cases, swine flu causes a mild illness which people recover from without any medical intervention," he said.

More than 3500 people have been diagnosed with the potentially deadly virus across Australia, according to Federal Government figures.

A Federal Health Department spokeswoman said on Saturday it was inevitable that the number of people dying with the disease would grow.

But she said authorities remain pleasantly surprised that the virus had not turned out to be as potent as originally feared when the first cases outside the source country, Mexico, were diagnosed in April.

"We're very fortunate that it has turned out to be a relatively mild disease," she said.

The spokeswoman said while the usual seasonal influenza normally affected older people, swine flu was "definitely one that's concentrating on younger people and children".

"That's because it's a brand new strain and the (younger people's) immune system has not developed," she said.

"It has been around the world targeting young people and those with underlying respiratory conditions."

With the development of a swine flu vaccine at least two months away, people with existing medical conditions should still consider themselves most at risk, the spokeswoman said.

"What we are seeing in this pandemic phase we call 'protect' is the virus concentrated among people who are vulnerable, who have pre-existing conditions such as respiratory diseases like asthma and others like diabetes which weaken the immune system," she said.

"There is a very important message for people with underlying conditions to seek medical help if they have any flu-like symptoms."

WA chief health officer Tarun Weeramanthri said it was important to remember human swine flu was a mild illness for the "vast majority" of people.

Dr Weeramanthri said people needing medical advice should telephone their doctor or healthdirect Australia on 1800 022 222.

PERTH AAP