Kiwis are being advised to take extra precautions to prevent the spread of nasty stomach bug and flu strains this winter, both of which are causing havoc overseas.

A new norovirus strain that is sweeping through Europe and North America has been circulating in New Zealand for nearly a year.

More than a million people in Britain have been struck down with the Sydney 2012 norovirus strain, which was first identified by Australian scientists in March.

Environmental Science and Research virologist Joanne Hewitt said the strain - which is no more contagious than other variants - was now predominant in New Zealand.

"In mid-September we started to see a large increase in the number of outbreaks associated with this particular strain.

"That continued in October and November and we're still seeing it now."

People who become infected will be very sick for two days, with severe diarrhoea, violent vomiting, headaches and possibly fever. They should stay away from hospitals, rest homes, schools and workplaces for 48 hours after symptoms cease and carry out stringent hand washing.

New Zealand is also preparing for a second round of a deadly flu strain that has killed about 20 children in the United States and put thousands of people in hospital during the northern hemisphere winter.

The nationwide epidemic has affected 47 states, with New York state declaring a health emergency this week after 19,000 people were struck down with the virus. California, Mississippi and Hawaii are the only states without widespread flu.

A "very similar" H3N2 strain was circulating in New Zealand last winter, Immunisation Advisory Centre director Nikki Turner said.

Fit and healthy Wellington mother Catherine McIlroy, 49, died from organ failure after contracting the H3N2 virus. She had not received the influenza vaccine.

H3N2 would be one of three strains in this year's influenza vaccine that would be available in March, Dr Turner said.

It was also in last year's vaccine, but this year's version would be a "better match". Complacency, a shortage of vaccines and not being prepared for the early onset of flu, "had contributed to the epidemic in the United States".

Up to one in five New Zealanders are infected with influenza each year and about 400 people die. Flu vaccines are free for people considered at risk of severe complications, including pregnant women, people aged 65 and over, and anyone with health conditions such as heart disease, diabetes and most cancers.

- © Fairfax NZ News