We received some excellent advice on a land tour, including particularly from Julie @OzKiwiJJ , ahead of our cruise (AKL to SYD) in Jan '19. We planned around visiting places we wouldn't visit on the cruise.
Based on the advice we flew into Christchurch & picked up rental. We stayed in ChCh , Mt Cook, Queenstown (3N), Franz Josef, Westport, Picton, crossed Cook Straight to Nth Island on the ferry, & stayed at Tongariro (2N), Waitomo & then Auckland ahead of the cruise. All accommodation & car rental was arranged thru a NZ tour company, which worked well for us.
So 12N for maybe 2/3 of Sth Island & 1/4 of Nth Island. We made the most of the time, but another week at least would have been better.
Good luck - it is a wonderful place to visit.