New Zealand has 3 festivals in this guide, spread across 3 cities. They run in February and December, and festival-day temperatures range from 24°C (75°F) to 24°C (75°F), with nights down to 14°C (57°F). That range is the reason there is no single "New Zealand festival outfit". The answer changes by month and by site.
Conditions are moderate across the board, so the day-to-night temperature drop is the thing to plan for rather than the weather itself.
The ground matters as much as the sky. You will find beach and open field, which puts flat sandals you can rinse, plus trainers for the night stages at the top of the list for most of these events. 3 of the 3 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans camping, beach, party and y2k. Rhythm and Vines is the largest of them at around 25,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.