For Christchurch this is a really good price as normally they are over £600. Flights are with Cathay Pacific/Qantas. I noticed some have a long stop in Hong Kong of around 14 hrs but I'm unsue if you can add longer stop over.
For a few pounds more you can get flights to Wellington (the North), but I personally think that the Christchurch flights are better value.
The time period that seems to work for this deals are around May and June and whilst this may not be the best season if you can look at early May you should still get some good weather.