What kind of supplies? I have found the diabetic care pretty good with a couple of provisos
1) With a population of only 5 million state funded medical supplies tend to be quite restrictive eg blood testing meters are restricted to caresens, though you can have as many strips as you want/need (including blood ketone testing strips).
2) Lantus is the only long acting basal available
3) But my consultant keeps offering me a pump....
4) Both dexcom and libre are available, but you'll have to self fund. Likewise, there's nothing to stop you buying strips for your favourite glucometer, but you'll probably have to import them from overseas.
There's a small fee for an appointment with a GP but hospital diabetes care is free (I get an annual visit to consultant, DN and dietician and have a number to phone the DN for queries). My GP would also do an annual check if I wanted one, and I get plenty of hba1c checks.
Likewise there is a prescription fee (with a yearly cap) based on number of items ordered, not cost of said items, so your diabetes is funded by the state, you just don't qualify for free prescriptions like the UK.
Home - PHARMAC | New Zealand Government
is the list of what medicines the NZ government will pay for (and what is available in the country). They do gradually add more items, but it's a very slow process.
You'll want to register with a GP, which is fine if you're planning to be resident in NZ. (When my father visits from the UK I talk him to a drop in clinic attached to the local hospital for care and scripts, as he's not eligible to sign up with a GP. But they charge $50 a visit.)
It's a lovely country, though you'll have to wait a while before travelling here (I doubt they'll be opening the borders to the UK till the end of 2021.) At the moment all international travelers have to quarantine in government policed hotels for 2 weeks and the number of spaces is very limited, we haven't seen our adult children (who live in Sydney) since last Christmas and will be spending our first Christmas apart from them this year.
I've lived in the UK and Australia before moving here in 2017, and I'm happy with the diabetes care, though I would prefer it if pharmac had a wider range of diabetic supplies.