I live in Dublin, Ireland and have just come back from a 2 week trip to the South Island.
A lot depends on what type of accomodation you will be using. For hotels / motels you will need to base yourselves in the larger towns. For campsites, trailer , etc,,,, the options grow exponentially.
I arrived in Christchurch.... stayed beside the park for 2 nights. I would have been better (reduce travel) to arrive in Queenstown. I wanted to stay in an urban environment for 2-3 days, to get familiar with New Zealand, recover from jet lag, organise mobile phones, maps, generally climatise.
If staying in Christchurch, Lyttelton Harbour is just South of Christchurch. I got some spectacular dawn shots from the ridge on the northern side of this large fyord like inlet. (see link below). You should drive this road during the day and use it to plan a dawn / sunset shoot if staying in Christchurch.
I constantly underestimated how long it took me to travel from point a to point b. The roads between major towns are good (not motorway), but if I estimated 2 hours it usually took 4. You may end up driving on a lot of unsurfaced roads. Most roads down to lakes and campsites are unsurfaced. You will get used to driving on these and finding the right speed. You will also recognise when an unsurfaced road is just too challenging. I found the Lonely Planet book to be extremely useful.
If possible, base yourself in Milton Sound for 2-3 days. It is 70 to 80 km/miles from Te Anua to Milton Sound. The closer you get to Milton Sound the more spectacular the locations. It is much easier to base yourself in Milton Sound than Te Anau and driving long distances to get to the choice locations. Accomodation is limited and difficult to get in Milton Sound, so I would get this reserved and then schedule other trips around that. There are loads of campsites between Milton Sound and Te Anua, but almost no hotels..so plan in detail (and in advance) your accomodation needs while in this area.
Good places also as bases are Te Anau, Queenstown and Wanaka. Study the main roads of the island carefully. Recognise that Milton Sound is at the end of a very long cul de sac. A really beautiful cul de sac.
I travelled 3500 kms in the 2 weeks while I was there. I did not have time to visit lots of locations I would have liked to. My conclusion was that for a quick tour you need a min of 3 weeks. However, you can do an awful lot in 2 weeks. I think quality time in fewer locations will be better than trying to get to all the landmark sites.
Here is a link to a few of my images taken on pbase. http://www.pbase.com/gnits/nz
I am more than happy to answer any specific questions you might have.
Enjoy your trip.