As Australians face a "horror" flu season, and "a fair bit" already here, New Zealanders cannot afford to be complacent, experts say.

Less than a month into the official flu season, early signs show we cannot assume it will be another mild year, Immunisation Advisory Centre director Dr Nikki Turner said.

The number of flu-like illnesses reported in the first half of the month remained on par with other years but a higher-than-usual proportion of those cases were being confirmed as influenza rather than other respiratory illnesses, Institute of Environmental Science and Research data showed.

Nearly 35 per cent of