Whanganui medical staff are bracing for an influx of influenza cases with strains such as the "Aussie flu" due here in two weeks.

Now is the time to get vaccinated they say.

A new vaccine has been updated to counter several strains of influenza, including the "Aussie flu".

A clinical nurse at the DHB, Jacqueline Pennefather, said the number of influenza cases was starting to increase in New Zealand.

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"We would expect influenza virus circulation to increase ... in the next few weeks," she said.

"There are other viruses circulating in the Whanganui community. These include Rhinovirus, which is still the most commonly detected respiratory virus in the community, and Adenovirus."

Rhinovirus causes upper respiratory tract infections, among other effects, while Adenovirus causes severe respiratory tract infections.

Neither of them were covered by the influenza immunisation, Pennefather said.

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

Flu vaccinations were at their highest in the last six years, with more than 1,294,907 New Zealanders vaccinated so far - an additional 77,413 more than last year.

District health boards have raised concerns about their ability to cope should this year's flu prove more virulent than normal, with beds already at capacity in a number of places.

Whanganui's occupancy rate has increased on last year, up from 84 per cent in June 2017 and January 2018 to 89 per cent as of last month.

The DHB was pushing a campaign to reduce unnecessary visits to the Whanganui Accident and Medical Centre and GPs.

"While a cold might take a day to develop, influenza can do so within a few hours and with an accompanying fever (more than 38.6 degrees Celsius) and muscle aches," said Chester Penaflor, a health promotion officer at the DHB.

"Most colds last a week or two at the most, and in general, you probably won't need to see a doctor. Self-care such as getting plenty of rest, drinking lots of fluids and avoiding exposure to smoke is what you need to do."

Penaflor said symptoms that should lead to a GP visit included: skin rash, an increasingly painful earache, a sore throat that gets worse, difficulty with breathing and/or chest pain, high fever, chills and headaches that last several days.

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

National Influenza Strategy Group spokesperson Brenda Saunders said it took up to two weeks after the vaccination for the body to start developing protection against the flu.

"We would recommend immunisation now, especially for those people 65-plus, those with ongoing medical conditions and pregnant women because they are most at risk of developing complications from influenza".

The Institute of Environmental Science and Research (ESR) monitors flu activity through visits to general practitioners, health line calls and hospitals.

ESR's New Zealand influenza intelligence report revealed flu had been detected in GP visits more frequently in recent weeks and influenza virus circulation was expected to increase in the next few weeks.

Saunders said some people may have the flu and not realise.

"Four out of five people infected do not show symptoms. This is called an 'asymptomatic flu'."

Saunders said the figures showed only how many vaccines had been distributed to medical facilities but "we would like to think 1.3 million doses of vaccine have been used".

The vaccine was not 100 per cent preventative, but it was the best protection against contracting the flu this winter, Saunders said.