By all means go down to Christchurch if you have a plan. Probably best waiting until volunteer organisations are able to get coordinated. Just stay out of the central city - sightseers are hindering the rescue efforts. That can not be stressed enough.
There are plenty of ways you can help without going down:
http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10708187I hope everyone down in Canterbury is alright, please know the country is there for you and is feeling your pain.
Many areas are inaccessible by road and have no power, water or sewerage, so things are going to get worse before they get better. If you are going to come here for family reasons then try to make sure you can actually get where you need to first. If you have family that can come to you that may be a better option as a lot of people just want to leave town.
We headed into Sydenham to check on my elderly uncle today and the devastation is just incredible. Yet barely 20km away we have no damage at all. Feeling very lucky just now.
Thanks for your best wishes FP, it's sure appreciated.