North Island is the only one with volcanoes AND gets tremors.
It's much busier of course, with Auckland and Wellington located
there, otherwise I can't comment as I haven't been there yet.
Only the Canterbury area is still affected by aftershocks in the
south. Christchurch is a mess right now but it's still a good
launch pad to see the Fox, Tasman and Franz Josef glaciers,
Queenstown, Dunedin, Kaikoura etc.
You'll see kiwis and dolphins on both islands.