Nah, you'll be grand!
North Island is beautiful, civilized, interesting - and 4 seasons in one day have the advantage of a washed-clean atmosphere.
Here is a
world light pollution map, currently centered on Coromandel.
For volcano hunting, Auckland is great, too.
My favourite city to live in was Wellington; (one of the) the dark sky location(s) there is Red Rocks where you might hit on a crowd learning Astrophotography from
Mark Gee.
If you go to Welly, be sure to take
Bus No. 14 from the city centre to Kilbirnie (and back) - it's a roller coaster of a bus ride up and around the steep cliffs
.
And if you can spare the time, take a ferry to the South Island (and back). Just a day trip without your car. That ferry ride is one of the most stunning normal public transportation things I ever used. Breathtaking beauty spreads before your eyes when the ferry enters the sounds on the top of the S.I.
The bluest sea and the greenest steep hills you can imagine.
And when you get back, the view of pretty Wellington from the ferry gives you an idea of why it's really a privilege to be a Wellingtonian.
If you're not going to Welly but are staying in the Northern part of the North Island: your travel guide surely mentions the Kauri forests, Coromandel, Matamata and the geothermal Wonderland around Rotorua.... Dark skies are only ever 2 stone throws away. Check the map
A very good choice to spend time in NZ! You'll love it!