Just did a South Island road trip, mostly not pre-booking anything.
Booking.com, etc are good to find places with vacancies. If you then contact the motel directly, you can sometimes get lower prices. Sometimes you get higher prices, in which case mentioning that their vacancy on booking.com has a lower price got a reduction.
Best place we got wasn't even on the Net! We found Bannockburn Vineyard Cottage listed on a local map provided by the Cromwell iSite.