Welcome to AFF Jay95
Yup anywhere in Oceania on NZ (only relevant UA partner).
20,000 miles is round trip in economy.
In Oceania NZ flies to NZ, Australia (SYD/MEL/BNE/CNS/ADL/PER), Fiji (NAN), Cook Islands (RAR), Norfolk Island (NLK), Noumea (NOU), Niue (IUE), Tahiti (PPT), Samoa (APW), Tonga (TBU), Vanuatu (VLI). Some flights to NAN, RAR, PPT, APW & TBU are on 767 (ie longhaul a/c), rest are shorthaul (mixture 733 and A320). Note some destinations are flown only once a week.
You might find Air NZ route map and online timetable useful.
United Mileage Plus miles if no activity for 3 years.