Welcome to NZ Notsaf!!
North Island - can be warm, a little windy and a cool breeze at night but very temperate. Definitely a sweatshirt or light knit but mostly sundresses, shorts etc are fine
As you move South it can be both colder and hotter. If hiking I strongly recommend merino tops and a windbreaker/raincoat and a wool hat. It can be cold at altitude.
The sun is intense here - make sure you cover up from it. I love my linen long sleeve tops with shorts in summer at the beach with a wide brim hat.
Quite honestly, I would only bring a gear raincoat, not a trench. I think it will be too warm and bulky - they do get worn here but more for transitional seasons or even winter in the North.
Our weather is unpredictable and can change daily!! Auckland will be the dressiest area, Rotorua is very casual and the South Island is more outdoorsy although people will dress up for wineries and restaurants. Jumpsuits or sundresses with a light topper, wide leg pants and wedges - that type of thing.
For the wedding I would take a light jacket or cardigan or wrap - my gold bomber is a workhorse for these types of things. A dressy cardigan could work too.
In terms of where to go and what to see
- in Rotorua the redwoods and the lakes are lovely - my favourite is Tarawera but the Blue Lake is a nice place for a swim or walk. The Mountain bike trails are wonderful - the Dipper is a lot of fun!!
- you can't really go wrong in Queenstown/Central Otago - Glenorchy/Arrowtown/Gibbston Valley/Clyde are my favourites - walks, bike trails etc... If you can get there Milford Sound is wonderful, we went there two years ago - but it's a LONG drive from Queenstown.
Feel free to ask any more questions!!