Reasonably, assuming you're familiar with NZ road conditions/design, the weather, traffic, sheep/cattle delays (no, really), one way bridges, one way bridges with trains, etc. If you're not familiar with those and/or want to stop for photos or anything then you can easily add half as much time again.

By way of example, I've done Christchurch to Nelson in 3.5 hours via Springs Junction. That was straight driving and no messing about (& considerable disregard for the speed limit). I've also done it in 9.5 hours with stops for photos, moderate traffic, 2 stops at tea rooms for a snack and rather nice and awfully relaxed lunch, etc. Google maps says it's something like 5.5 hours.