It’s best to go here in the summer. The West Coast rains a lot and it can be miserable, cold and windy there, even in the summer. It’s not a place you want to be in the winter when it’s raining and cold. It can be quite dramatic and moody in the winter and not impossible to do, but it’s a much more pleasant experience when the weather is nice.
Bring Mosquito or bug spray. The West Coast has a lot of bush and water and that’s a breeding ground for sand flies and mosquitoes. Sand flies are smaller but make nasty little bites. They’re particularly bad at the Blue Pools and Hokitika Gorge which are both essentially rivers in the middle of bush.
Take your time. The roads on the West Coast are good but a bit winding and there can a lot of campervans and slow driving tourists in rental cars. There’s plenty of places to pass so just take your time as by the time you find somewhere to pass you’re probably going to find a spot to want to spot and take photos. You can spend a night somewhere along the way, either in Hokitika or Fox or Franz Joseph Glacier. Greymouth is much bigger but not much to do here although Punakaiki is only about 30-40 minutes away.
There’s a few ways to get back to Christchurch. I usually take Arthur’s Pass. There’s a few things and places along the way but it’s really quite an easy drive and not many cars on the road. I’d do it at night, hard on the eyes but less traffic. You could check out Hamner Springs on the Way and even stay a night there.
There’s not many hotels at some of the smaller towns like Fox Glacier and Franz Joseph Glacier so if you’re the type to just show and do shit last minute, don’t get your hopes up of finding a hotel last minute especially during high season. The towns can be few and far between so it’s not like you can just drive 5 minutes and find something else. Book everything in advance, you can probably do the West Coast over 2 days, staying the night at one place. If you love walks and spend time at places, then perhaps 3 days, but don’t think you’ll need more than that to cover most of it.