Can you be spontaneous with overnight accommodations in New Zealand?

We were travelling in a campervan in May so very different circumstances.
We did book into a backpackers in FJ about an hour before arrival. Also booked our final night hotel/motel in C'church the night before.
I doubt you would have many issues in the places we visited last year on the South Island. Only places crowded with tourists were Tekapo, Queenstown Wanaka and Milford. Twizel (great base for Aoraki/Mt Cook) and Te Anau (great base for trips to both Doubtful Sound and Milford Sound) didn't give the impression of being chockers with tourists.
We are off to the North Island end of April but again it will be campervan not hotels.

That time of year you should be well clear of the crowds that tend to fill up the small towns during the Chinese New Year period, which usually ends by the middle of Feb, so you should be fine as long as you're willing to settle for "whatever is left" when looking for accommodations.
Know too that the further south you go (am thinking Invercargill and Stewart Island) you're more likely to encounter unsettled weather by the beginning of March, while the North Island will be terrific.

Check the New Zealand school and public holiday schedules. I was advised (2019) that advance reservations were often necessary if there are New Zealand vacation periods as many locals traveled then. Perhaps that has changed, but I would check into it as I doubt that the tourist infrastructure (motel/hotel/lodging) has changed significantly.

Thank you for your replies. We plan to rent a car and tour the South Island for the first 2-1/2 weeks, then fly to Auckland and rent another car for the remainder of our vacation. We are concerned about accommodations being full but we love to "wing it". We did a road trip from Melbourne to Adelaide, Australia and had the best trip ever. I will check the school holiday schedule. Good advice!

I am here now, but arrived with all 5 of my weeks booked. The worst rush of the vacation time seems to have passed and if you are flexible on quality and pricing (and maybe flexible on actual location), you can probably be spontaneous. Per my conversation this morning, the exception might be on the long weekends for different provinces celebrating independence (not something I was aware of). Auckland’s is coming up next weekend and things near the coast, etc. book up quickly. Different weekends in different places. I haven’t made it to the South Island yet.

If you have particular things you want to do or places you want to see (and stay), I wouldn’t leave those to chance unless budget isn’t a problem. But otherwise, probably possible.

Thank you Texas Travel Mom for taking time out to reply from beautiful NZ! There are certainly advantages to booking ahead. Have a lovely time!