New Zealand - South Island

My wife and I (60's) are planning to visit NZ for 3 weeks and Tasmania for 2 weeks. We would like to do three treks in NZ, while we are there; 3 - 6 day in length for each. We are experience hikers and carry 30-40 pound packs at altitude, so we feel we are up for challenges. We have backpacked around the world and are now planning a New Zealand / Tasmania trip. We are up for camping, if required, or hiking/tramping hut to hut and cooking our own food! Guide is not necessary, but sometimes worth it to learn more about the area. Any suggestions are welcome. We like it where there is less people and the challenge gives you amazing scenery!