Congratulations Steph58 ^ Looks like a nice trip.

Havelock North to Wellington. I'd probably take the Wairarapa route - perhaps a bit more boring scenery generally than through the gorge, but nice countryside. A couple of nice towns to stop along the way (and good wines in the Martinborough area - small detour required). Less traffic this way.

Nelson to Christchurch - I'd take Lewis Pass. Not only does it give some nice subalpine scenery which you don't have anywhere else in your trip, but it also avoids doubling back through Blenheim. If you have time, a short detour to Hanmer Springs is worth a visit - quaint town and lovely hot pools, in a great forest setting. When you reach state highway one, turn left for Waipara a short distance away and some more wineries.