The cheapest combination I found seems to be from Mel $196 but there aren't that many flights. Pretty much $206 or $216 return. Sydney is a few bucks more expensive.
Travel Dates:
$99 Sydney to Christchurch: 1 Aug - 19 Sep 2012,
$99 Melbourne (Tullamarine) to Christchurch: 1 Aug - 19 Sep 2012,
$99 Gold Coast to Auckland: 1 Aug - 19 Sep 2012,
$99 Sydney to Auckland: 1 Aug - 19 Sep 2012,
$99 Melbourne (Tullamarine) to Auckland: 1 Aug - 19 Sep 2012
Hmm. The return leg used to be cheaper than $99 but now is more expensive.