Today is a good example of the need to have buffers in your plans rather than driving long distances every day. (Note this doesn't happen often but poor weather can sometimes close multiple highways simultaneously.)
The main road between Christchurch and Picton is closed for several months following last years earthquake. The alternate (much longer) route through Lewis Pass was closed for several hours. Also the roads between Nelson/Blenheim and the west coast were all closed. The road between Christchurch and the west coast is closed for a couple of days (perhaps 3?). The main roads along the west coast were also closed in several places. Together these closures account for a significant portion of the south island highways.
Most of these closures are very temporary - several hours to a couple of days. But they could cause problems with tight itineraries for some.