Having just come back from a fact finding mission, here are a couple of estate agents websites
In Chirstchurhc (but also nationally)
www.harcourts.co.nz
A very good website and their policy is to market most properties with prices, rather than as a price by negotiation which is sooo infuriating. The chap in the Halswell branch was very helpful with answering our questions about the housing and what areas we were likely to find something suitable in. They are also the biggest estate agents in NZ, so worth a gander

The main publication that we found was the Realtor, it has an online version here
http://www.realtor.co.nz/

In the Dunedin area
www.edinburghrealty.co.nz
I can't say a recommend them as we didn't find them very helpful at all, but it may just have been that one sales guy that we spoke to, so have included it anyway.