A campaign to vaccinate hundreds of thousands of young adults against the measles has barely made inroads, four months after its launch.
Only about 4000 have been immunised since the high-profile government campaign launch in July.
About 300,000 people aged between 15 and 30 needed to be immunised to help prevent another outbreak like the one that hit the country last year.
The head of the Immunisation Advisory Centre, Nikki Turner, said the gap left the country very vulnerable.
Measles was on the rise internationally because many countries had turned their focus to Covid-19, she said.
"Measles is one plane trip, one boat away from New Zealand. While we've got borders under pretty good control at the moment we're okay, but it'll be really any moment now that we'll be seeing a measles case arrive back on the shores of New Zealand," she said.
The gap in the country's immunity is easily enough for an outbreak to squeeze through, as it did last year.
Then, nearly 2200 people were infected, most of them in Auckland - and a smaller, earlier group in Canterbury.
Canterbury DHB needed to vaccinate 35,000 people but had only reached about 600 so far.
The DHB's Child, Youth and Family manager Bridget Lester said it was tough trying to get the job done in the middle of a pandemic.
It had been difficult to get stretched GPs and pharmacies fully on board because they were so busy with the Covid response.
And there had been a lack of publicity material, with posters only going out around the region this week because they did not get the branding from the Ministry of Health until about a month ago, she said.
"It has been really frustrating to roll the programme out but we also have to acknowledge the environment we're in. We've got a pandemic and we've also had an election so with those barriers it's been a challenge," she said.
Christchurch and Auckland GP Api Talemaitoga said those working in the health system were tired.
It was hard to find enough staff to do the labour-intensive work of going through clinics' databases to find anyone who might need to be vaccinated and trying to contact them then vaccinate them.
"I think there is a general lethargy. Everybody is tired. And we've been working really hard over the year and it's been a stressful year for everybody, for primary care, for the community," he said
But the work was critical, he said.
He hoped GPs practices could find the little bit extra they needed to do the work, but also that they got better support from DHBs - like with help identifying who needed a jab, or with organising special vaccination sessions.
"The real risk is that we reintroduce another community spread of measles and we'll be scrambling then to try to do things after the fact," he said.
Canterbury DHB said most high school students had been well covered and it was working with tertiary institutions and companies that employ large numbers of young people.
The Ministry of Health said it was supporting DHBs with promotions on radio and social media. Early next year that would be extended to TV.