Flying in the Mainland (South Island) can get real interesting when there is a strong Nor'wester blowing. The turbulance can be extremely violent. You don't have the same problem in the North Island since the tail end of the Southern Alps tapers off as it goes up and to the east side of the island. Wellington can have similar excitement when there is a wind blowing around there. My head has made contact with the roof several times when departing Wellington during a good blow.
Apart from the Nor'wester, flying during the mentioned period is usually really good and if the Nor'wester isn't too strong that doesn't present much of a problem. I cannot remember any time when I had to cancel a flight during the spring/summer months. Winter in the south is however a dfferent story. Cannot comment on winter flying nother of Cook Strait.
Getting an aircraft usually isn't a problem unless the club has competitions or a fly away has been organised. Weekends can be an issue but if you book early enough then you can get around that.
Aircraft availability can be ascertained by emailing the club and asking them. There are also some private aircraft for hire and it is just a matter of finding them and having the appropriate number of hours for their insurance requirements.
It's a great place to fly but them I do have a small degree of bias (that could be straightened if I applied a bit of rudder trim
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