What kind of speeds do you see with Spark (and at what price)?
Most of rural NZ has zero coverage. And i'm not talking very rural, you can still be IN WELLINGTON CITY and still have no coverage. I lose coverage sometimes biking along the waterfront. If you go 30 minutes outside of the city centre, there's basically no coverage. Along major highways, like driving from Wellington to Auckland, MOST of the route has zero coverage.
Contrary to the parent I've found coverage to be fine, and haven't noticed losing coverage in places that I'd expect it.