The west coast of the South Island is great for cycling. The roads aren't too busy and there are some lovely flat to undulating stretches with the odd climb to keep you alert. If you're into wine I can recommend Central Otago, and there's even an organised race you can take part in:

http://www.cromwell.org.nz/events/su...e-classic.aspx

I'm biased about this one because my father-in-law has a vineyard not far from Cromwell in Bannockburn!