Late April early May are best time for fall color in the South Island, the North can be a good 4 weeks later. Your window is quite small, if we get a winter storm coming through the wind and rain can kill the color quite quickly. Remember native NZ forest is ever green, so the fall color is only from exotic trees generally found where there was significant settlements in the early 1900. Best places tend to be in the South Island as there is far more seasonal change the further south you go. My recommendation is Central Otago / Southern Lakes district, essentially Queenstown & Wanaka for deciduous tree color, maybe take in Alexandra & Cromwell too, All within a 2 hour radius. Christchurch has excellent parks with good color too but has of course been severely trashed buy the recent earthquakes. You could consider a photo tour with Mike Langford & Jackie Rankin out of Queenstown, check out www.qccp.co.nz
Depending on the time you have available the biggest density of " photo opportunity " is in the South Island. A visit to Fiordland is essential, and yes, the Glaciers, Fox Glacier and Franz Joseph Glacier, on the west coast are awesome too, but you will need a heli tour for access. Other stunning places are Lake Pukaki ( Mt Cook ) and Lake Tekapo both in the Mackenzie Basin area.
Get that credit card out and book it now !!
Here's a pic from Queenstown to inspire you along ......