Australia's chief medical officer says there has been a significantly higher number of flu cases than normal in people who have been vaccinated.
Professor Brendan Murphy said this season's vaccine, selected by the World Health Organisation, gave "moderate to good" coverage of the viruses, but it had not been as effective as previous years.
The reasons for that are still unclear.
"We have been notified of about 100 deaths from influenza so far this year, but we believe that to be an underestimation," Prof Murphy told reporters in Canberra on Wednesday.
At least 170,000 cases have been confirmed this season, about two-and-a-half times more than in 2016.
Most are linked to a particular strain of the influenza A virus, which has a predilection for people aged over 65.
Prof Murphy said it wasn't a very nasty flu, but of "moderate severity".
"It's a bigger season than usual, but we know those cases are just a fraction of the total number of people infected with flu," he said.
"The reason we've seen so much evidence is probably due to the number of cases rather than it being more severe than usual."
While there had been an increase in hospital admissions, fewer cases have required intensive care.
Authorities are looking at alternative vaccinations, particularly for use in the elderly, for future seasons.