Odysseus has it correct about staying out of Christchurch if possible… I work for a local authority in the South Island and the message we are giving is that the damaged infrastructure is really struggling to cope with those resident in the city already, so extra visitors to the city, however well intentioned, can cause further strain on limited resources…
However ,perhaps if you were to stay outside of the city, but within driving distance (eg Ashburton?) and you could find accommodation (local towns are pretty much at capacity for accommodation through people fleeing Christchurch), then maybe you could volunteer at http://www.sva.org.nz/help/offer/ which is the University of Canterbury’s Student Army. They are working mainly clearing rubble and silt. You would need to take your own wheelbarrow etc as lack of wheelbarrows is really slowing the work.
Awesome that you want to help – I am sure that if you can’t help with manual work, a donation of dollars to the Red Cross or maybe to a foodbank in Christchurch or local towns would be very welcome! :-)