For driving - it really is a matter of YMMV. I found driving Christchurch-Franz Josef and Franz Josef-Queenstown rather doable, in large part because it is so scenic, and we split up the ~4.75 hour drives with a multi-hour hike in the middle. The only drive that I found to be quite difficult was driving from Martinborough to Tongariro National Park, as you have a lot of flat, rolling hills with very little to look at until you get closer to the park.

That said, 9 days is pretty tight for the entire country. I did both islands in 2 weeks and that was a bit of a stretch (although I did one-way car rentals, so did not have to worry about returning to a starting destination point).