A vaccine to fight this season's set of deadly influenza strains has arrived in New Zealand - but people with poor immune systems should wait as long as possible before getting their shots.

The vaccine arrived in New Zealand in late March for the private market but national suppliers are still distributing it to general practices around the country, Associate Professor Nikki Turner, director of the Immunisation Advisory Centre (IMAC), said.

It could take till the end of the week before all GPs are ready to go, but people can start contacting their GP now to ask about getting their shots.

The flu vaccine is free for people who are most at risk, including pregnant women, people over 65 and children with a history of respiratory illness.

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"The flu season normally starts in June or July but you need to get it two weeks before that for immunity, so [people] should get it any time in the next two months," Turner said.

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However Turner cautioned that people with weaker immune systems might benefit from delaying their visit till May.

That's because they have a weaker response to vaccines, and the effect could wear off before flu season hits.

"People whose systems are not so vigorous - the elderly, or people with medical conditions - it might be better for them to get it in May rather than April," Turner said.

"For healthy people who have a good immune response, getting the shot any time is fine."

That includes pregnant women, who have a strong response to the vaccine. Turner said it was particularly important for pregnant women to get the flu shot as it would also give protection to their newborn babies.

The past year has seen several deadly strains of flu sweeping Australia and the Northern Hemisphere. There are fears any of those strains could prove nasty here as well.

The situation in Australia was made worse because the vaccine for a vicious strain of Type A flu - AH3N2 – was not very effective. Death rates from Aussie flu last winter were the worst in years.

The latest New Zealand vaccine has been updated to fight four strains of the flu that have been causing trouble overseas - two "type A" and two "type B" strains.

But it's still not clear whether it will be effective, or how nasty the flu will be when it gets here, Turner said.

"We have changed the content of the flu vaccine so we're hoping it will be a better match. But flu is really unpredictable and we always have to be prepared for a bad season. Only God really knows the answer to how it's going to go.

"The last two seasons were pretty mild and we got off pretty lightly but even that fairly average season was really challenging for our hospitals."

Auckland's hospitals had their busiest winter ever in 2017, but admissions spiked even higher over summer, which is normally quiet.

With beds already at capacity, DHBs have raised concerns they will not cope if this year's flu proves more virulent than normal.