Free flu vaccines for children aged under five are now available, as the health system struggles with the worst outbreak in years.

Associate Minister of Health David Seymour was in Auckland with Minister of Health Simeon Brown where they announced the changes to flu vaccine funding criteria.

Pharmac's extension of free flu jabs for children between six months and five years of age begins on Tuesday - rather than from 2027 as announced last week.

The ministers said the decision was in response to patients asking for vaccines sooner, with more than 260,000 young children now eligible.

Health Minister Simeon Brown (left) and Associate Health Minister David Seymour. Photo: RNZ

"Young children get sicker from the flu than most people realise, and they are more likely to end up in hospital, with the youngest infants at the highest risk of all," Brown said.

Children under five years old have had the highest rates of hospitalisations during this year's flu outbreak - one of the most severe in years.

"Flu season puts a lot of pressure on households, workplaces, schools, and the health system. Today's decision will give families the support they need to manage the flu this year," Seymour said.

Emergency departments had seen record numbers of people and struggled to cope with staff off sick.

The Royal New Zealand College of General Practitioners has welcomed the decision, after having asked the Ministry of Health and Health NZ for wider vaccine eligibility.

College president Dr Luke Bradford told RNZ prevention was the best way forward.

"Some of the most important health care happens before a patient ever needs to go to hospital. General practice is where preventative health care happens, and vaccinations are one of the best examples of how accessible, community-based care can prevent serious illness, reduce hospitalisations and keep people well."

Plunket acting general manager of service delivery Nat Hornyak said its telehealth line had received more than 200 extra calls a day during the spike.

She was pleased to see funding criteria widen in response to the health system's feedback.

"Absolutely delighted with the agility and responsiveness to what we're seeing in the data because those numbers are families and their tamariki in hospitals and unwell.”

Pharmac director of pharmaceuticals Geraldine MacGibbon told RNZ the government’s drug-buying agency had targeted those most at risk of hospitalisations when considering a response to the spike in infections.

Children under five showed the highest rates of flu-related hospitalisations - making them the priority group, she said.

"We had a look at our stock levels, we've been monitoring the activity of the flu, determined that we were able to support the widening access and moved really quickly once we had come to that conclusion.”

She confirmed the change would be permanent.