The plant hardiness zone map below, maps zones of minimum temperatures. These give an indication of whether a plant is likely to be able to survive in a certain location based on its minimum survival temperature. This map gives no indication of maximum temperature, humidity, light and soil moisture content, all important factors on a plant’s survival.
New Zealand Plant Hardiness Zone Map
landscape architect's pages 
With all the caveats you note to plant survival is this map in anyway useful?
It provides some (most probably the most important) of the information as to whether a plant will survive in a particular area. The caveats suggest additional information which should be investigated before making a final decision on a plant’s suitability for a particular area. I hope that helps.