Bring vs rent? (Coming from California)

Hello! My husband and I will be in New Zealand for 4.5 weeks starting Feb 1 2015. Many parts of the trip will involve hotels, B&B's, and for at least many days, staying with family in Christchurch. We'll be doing several day hikes and typical tourist travelling for most of that month, but have Milford Track reservations as well. So, for 4 days of a month long trip, we will need backpacks. For the rest of the trip, a backpack could be a perfectly good way to carry some stuff, but wont be needed. I dont know whether to bring mine or rent one there. I have 3 packs, from 44L to 75L. Mine are fitted to me, proven to be comfortable, etc. If I could fit all that I need for the Milford Track in the 44L pack, I think it could smush down small enough to go as carry-on for our flight. If I bring the next larger one, it becomes "luggage." I'm used to carrying a tent, cooking equipment, therm-a-rest......so I'm trying to envision just how much space I need for a trip like this in which I DONT need that stuff, but need to be extra prepared for crazy weather compared to what I'm used to. I also just dont know how common/easy it is to rent good equipment there. My husband only has a 44L pack, though I'm trying to get him to backpack more with me, and may just get him a 60L one like my other medium pack...... I'm also not sure if I should bring or rent a sleeping bag, but I think that will get brought, since a few of our other nights will be spent on nice people's floors and in the back of a truck..... Any opinions? On one hand it seems silly to rent things I own, but I dont know how much sense it makes to bring something from here I only need for 4 days. Since my brother lives in Christchurch, and will be going on the Milford Track with us, we have the luxury of his truck for transportation and storage of the stuff we dont carry. We are very excited to be coming, and I will have lots of other more interesting questions later, but for now I am focusing on the practicalities! Thanks, Anne