How much experiance do you have tramping? If you don't have much, I wouldn't. There's just sooo much that can go wrong.
If you've got the gear and have plenty of experience, I still don't really think I'd do it. I can think of a few potential hitches - firstly, if one of you will be carrying the baby, you'll only have one pack to carry enough gear, food and water for the two of you and bubs. You could go as light as possible (I assume you'd be planning on staying in a hut, so you wouldn't have a tent to carry?) but even then, carrying gear for two people is damn hard work, and babies seem to need a lot of extra stuff. It would be a really bad idea to skimp on things like polypros and warm clothes - you may not need them, but if you do, you can very easily die without them.
You'd also need to think hard as about contingency plans - what if bubs got sick or had an accident out in the bush? I know you always have to think about that anyway, whether you're taking a baby or not, but I guess I'd tend to worry more about a little one.
Maybe I'm getting soft in my old age

, but I think I'd consider the extra work and risk involved just too much. How about camping at a road end or in a DOC campsite where you can do a couple of really nice day walks? I think that's the option I'd go for, myself. I haven't done much tramping up north, but I know there are some lovely books that most libraries should have with neat walks.
Oh - and even though I probably wouldn't do an overnighter with a wee one, I think it's so cool that you're keen to do stuff like that with him. I think kids get so much benefit from getting out there right from when they're tiny. I did a tramp once in Nelson, and right at the end we climbed Mt Arthur.We passed a chap taking his 5yo kid up - te kid was a real plucky little bushman, he was really getting into it and I remember thinking that if I have kids I'd love them to be just like that kid.
Edited by Hopes