Would agree that Rotorua is not worth the stop - it is day tour distance from Auckland and so is filled with coaches full of Japanese tourists.
I wouldn't write off the North Island at all - there is plenty to see/do there. I guess it depends on what you are into - action sports, tramping/hiking, etc . . .
Hanmer Springs is . . . ok, but really not worth the diversion as it is not near anywhere in particular and the springs are pretty much like the springs you will find in plenty of other places in the country. My favourite springs area was the area around Taupo, especially a few motels to the south of the lake (not in the town) which have their own thermal baths where you can soak all you want. Stayed in one of them (the Rainbow Motel I think) and had a great soak one night - pitch black, cold air, looking up at the stars soak . . .
Agree that the Glaciers are well worth the visit, if only for the totally mind-blowing scale of the things. You approach them on foot, walking past boulders the size of your house just strewn around the place, and then you realise that the glacier face that looks near and small is actually *huge* and a long, long way away when you see the tiny people up against it in the distance.
To give you an idea of the distances involved - Auckland to Wellington is about 10-12 hours drive with the foot down all the way, but it is pretty pointless to do that since you would be better flying into Christchurch instead.
Milford Sound is . . . interesting, but definitely a full, tiring day to get there and back from anywhere even slightly near.
We were in Auckland when the Americas Cup was on there and it was somewhat underwhelming at the time, so I don't think there will be much left of it except for a few huge boat houses. It may have been a factor that Team NZ kept sinking their boats during the races, so the actual race days turned out to be a bit of a damp squib.
The national museum in Wellington (Te Papa) is well worth a visit and will give you a good grounding in NZ & Maori (and Pacific Islanders too I think) history.
We found the best things about NZ was the little things that we came across rather than the big events. Things like community facilities, tiny volunteer museums in every single town, and a great appreciation of history from everyone you meet.
Christchurch is a bit, well, small. We lived there for about 8 weeks and it never really grabbed me - there were some nice places nearby, but I think I compared it at the time (not in a positive way) to living in Leitrim. I have since learned that Leitrim has a lot going for it, so the comparison may not have been warranted.
In general, it will probably seem very tame after a few weeks of Australia, but that may not be such a bad thing.
Enjoy,
z
p.s. just checked my earlier posts about Rotorua and I see I previously recommended it as worth the visit despite the overly touristy atmosphere, so I guess it was worth it - I just can't remember what it was after 3 years, but I do remember the tour coaches.