Blenheim => Moreaki via Christchurch- is there much down this road?
The coastal road from Blenheim to Christchurch was a stunning drive after you cleared the inland section and hit the coast. The road snakes along right beside the surf in places with the odd tunnel to sharpen your senses. There was a sizable earthquake slightly inland in November 2016 (7.8Magnitude) which caused a lot of damage to the road and rail line. Until recently road closures or delays due to the repair were a factor...
but this handy link
shows the road open 24/7 now.
From Christchurch to Timaru the road is heavy traffic and allow me to suggest those f**kers are even more impatient than anywhere I've driven. There are roadworks going on on that stretch to add lanes and improve flow. That link I included will list them for you.
The coastal stretch down to Kaikoura is a more impressive drive than the Great Ocean Road in Victoria, I've done both. South of Kaikoura the road winds through coastal hills. It's narrow and windy all the way. The stretch south to Timaru is flat and predominantly straight as you're crossing the Canterbury plains. This is where I suggest the Inland Route through Mt Somers and Geraldine, a much nicer drive. You head out of Christchurch towards the West Coast and Darfield, then turn south.
From Timaru south (I've not done this drive for close on 30 years) was always a more interesting drive with the hills closer to the coast and the road winding through some nice scenery in places. Moeraki is definitely worth a look, it's quite a unique place.
EDIT... can't overestimate the tight, winding nature of the roads enough. It's slow going at times with a lot of heavy truck traffic to negotiate and Kiwi drivers are impatient. They'll sit right on your arse... it shits me.