A leading virologist warns that the predominant flu strain in New Zealand currently is particularly risky for elderly people - especially those with underlying conditions.

Six residents at a rest home in Whitianga have died after a third of residents and staff were infected with influenza A.

The director of the WHO National Influenza Centre at the Institute of Environmental Science and Research, Dr Sue Huang, said the H3N2 subtype of influenza A is the predominant strain in New Zealand currently.

"H3N2 hits elderly people particularly hard, especially those with underlying conditions," she said.

Dr Huang said they were still waiting to get samples from the Whitianga rest home to determine the subtype of the virus in that outbreak.

Influenza A (H3N2) was dominating the outbreak in the Northern Hemisphere at the moment, and was likely to be spilling into New Zealand, she said.

The flu vaccines this year, available from early April, would provide protection against the new strains in the Northern Hemisphere, Dr Huang said.