I never go around to doing a follow up post from my 2 week hike on the South Is last year. We spent three weeks there total in 2017 and went back for 2 weeks this year.
Anyway, all I used was Aeroguard Odourless in a small bottle (125ml). It is truely is odourless and non-oily, but not tasteless so wash your fingers before eating. Make sure to spray evenly over exposed areas, even between fingers as those buggars will eat you. I use it all the time in AU for mozzies, ticks and flies. It's not so good at flies here in Perth as the flies are too dumb and land on you anyway. They don't spend much time on you but land on you again anyway.
If you apply Aeroguard Odourless evenly, you will not get bothered with NZ SF's. Much like hungry Australian mosquitoes, NF SF's will bare the stench of repellent to find a spot that tiny is that you missed.
SF's can't bite through clothing, even thin layers. You might consider stockings/leggings if tramping in shorts. Do take a mozzie head net for some moments of relief when needed. You should be able to get one for a few dollars and weighs only 30g if that.
Also consider packing a a light mozzie/noseum net for day time sleeps/snoozes. Even something that covers head and shoulders would do. On the Nelson tracks we were all exhausted but couldn't lay down for a rest for many hours after arrival at camp or get eaten alive. I'll be taking my OR Bug Bivvy next time.
I also took along a Arcteryx Konseal which is similar to one of these:
https://arcteryx.com/ca/en/shop/mens/stryka-hoody and this was indispensable for protection against the elements and the SF's.