A GP in the thick of the measles outbreak says it's a difficult task to ration the vaccine.
Canterbury has received an extra 27,000 doses this week, but it's understood 100,000 are needed - and more aren't expected to arrive until April.
The last update from Canterbury DHB on Thursday said there were 28 confirmed cases and another 12 under investigation.
Dr Shelley Louw says the aim is to immunise as many unvaccinated and at-risk people as possible.
"If we don't get it, we could get travellers in from other parts of the world who haven't been immunised, and that just continues the spread. It's a concern if, for instance, Auckland can't get enough for their unvaccinated."
Some patients in Auckland looking for booster shots are being turned away, with vaccines being rationed to focus on groups most at risk - those between one and 28 who've never been vaccinated.
Pharmac has confirmed there are limited global supplies of the MMR vaccine, and its stock levels are "normal" for this time of year.
Dr Louw says the time and resources going into containing the outbreak are huge.
"Once you've had someone with measles in your waiting room, you've got to contact everyone that's been in there for three hours. Plus the hospitals wouldn't cope if we suddenly started admitting people with measles."
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She says her practise has about a third of what it needs.
"It's going to become more awkward when you run out of vaccines again. We might have missed some of those we've been texting to contact us for an appointment."
Wellington officials say they have enough to cover the region, which is so unaffected by the outbreak.
Health Minister David Clark on Thursday ruled out blocking unvaccinated children from attending school.