"This is not an ordinary winter. Swine flu is hitting young, otherwise healthy people …- they start with a sore throat, develop shortness of breath and within 12 to 24 hours have rapidly developed respiratory failure and are being ventilated," he said.

Dr Herkes said extracorporeal membrane oxygenation, or ECMO, was considered a last-resort treatment, but staff were "throwing everything" at the patients because they were young and relatively healthy.

Patients in respiratory distress are given anticoagulants by machine, and their blood is drained through tubing in their femoral or jugular veins. It is oxygenated outside the body, allowing the lungs to recover.

Three patients with swine flu had been taken off the treatment at Royal Prince Alfred in the past few days.

One was "sitting up talking, on a ward", but two were still critical and were being mechanically ventilated, Dr Herkes said.

Brad Frankum, a general physician and immunologist at Campbelltown Hospital, said he had heard anecdotal reports that "more people than ever before" were being treated with ECMO this winter. "This is of great concern because it would suggest that the number of serious cases [of acute respiratory distress] are threatening the capacity of the system," he said.

The deputy director-general of NSW Health, Tim Smyth, said yesterday five big hospitals in Sydney had now been designated to treat swine flu victims with ECMO, up from two a fortnight ago. He said about a third of swine flu patients in intensive care were needing this treatment, but there was still capacity to deal with the pandemic.

He said the department had stockpiled 130 new standard ventilators two years ago as part of the state's disaster plan and would open more intensive care beds on high dependency units if the number of patients continued to surge.

But Peter Collignon, a professor in infectious diseases at the Australian National University, played down the use of the machines, saying "this happens every winter - it just doesn't get publicity".

A spokesman for the State Government said elective surgery could be cancelled at Royal North Shore, Royal Prince Alfred and St Vincent's hospitals, and patients due for surgery would be moved to less affected hospitals.

"There's no move at this stage to move to a different [status] in our pandemic plan," a spokesman for the Health Minister, John Della Bosca, said.

There are many hundreds of flu deaths every year, but a senior health source said swine flu was likely to hit harder as there was no vaccine and no immunity.