Why do New Zealand's home Test series' tend to be played later in the summer than Australia's?

Seeing as they're both in the southern hemisphere, I've been wondering about this. The Test Matches in Australia usually start at the end of November and finish sometime in January (or February when two nations tour). New Zealand's games against England are all going to be back to back in March.

Granted that England have just finished a tour in India but they always tend to be late in the antipodean summer when we play the Kiwi's. If we add six months on to get the northern hemisphere equivalent, Australia's Test series corresponds to being played between May and July in the northern hemisphere whilst this series in New Zealand in March corresponds to being played in our September.