HONG KONG, Oct 9 (Reuters) - Swine flu sent 722 people into intensive care units during the winter months in Australia and New Zealand, far more than during a normal flu season, and many of them were infants, pregnant or obese, a study published on Friday found.

"Normally, we would only be seeing maybe 30 to 40 patients admitted to intensive care with the flu, it is actually very rare to come to intensive care because of the flu," said one of the researchers, Ian Seppelt, at the Sydney Medical School.

"But this year we saw a large number of younger people who were critically ill and who needed intensive care resources," he told Reuters by telephone.

The researchers, who published their findings in the New England Journal of Medicine, hope their findings can help northern hemisphere countries prepare for an expected onslaught from a second wave of H1N1 infections.

"We can give some indication what to expect in terms of numbers and resource requirements. What we saw was certainly a major stress on the intensive care system ... 20 percent of ICU beds were required by H1N1 patients," Seppelt added.

The study overed the period from June to August, the southern hemisphere winter.

While pregnant women make up only 1 percent of the general population in Australia and New Zealand, 66 of the 722 ICU patients, or 9.1 percent, were pregnant women.

Similarly, while 5.3 percent of the Australian population have a body mass index (BMI) greater than 35 and are regarded as obese, 28.6 percent of the H1N1 patients admitted to ICUs had a BMI of over 35.

An estimated 13 percent of people in the general population have asthma or other chronic pulmonary disease, but 32.7 percent of people admitted to ICUs had these underlying conditions.

"Those patients were over-represented ... that's quite disproportionate," Seppelt said.

The study, which pooled together information provided by 187 ICU centres in Australia and New Zealand, also highlighted other interesting details.

"Our data indicate that the greatest effect on ICU resources in a given region occurs approximately 4 to 6 weeks after the first confirmed winter ICU admission and the extra workload lasts several weeks," the researchers wrote.

Julian Tang, consultant and virologist at the National University Hospital in Singapore, and who was not connected to the study, said the second H1N1 wave would affect more people because none of them would have any existing immunity to this pandemic virus.

"Despite a very low expected case-fatality rate from this virus, the larger number of symptomatic infections may cause some additional burden on healthcare services," he told Reuters.

"Widespread vaccination may reduce the number of such symptomatic cases and therefore reduce the social and economic consequences of this." (Reporting by Tan Ee Lyn; Editing by Ken Wills and Alex Richardson)

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