In my limited and occasional experience the back roads behind Chch airport have been the doggers area of choic and are discreet. Sometimes a few of the plane-spotter crowd will let curiosity get the better of them so an audience of 15-20 can emerge.
I would suggest you post info only 24-36 hours before you plan to play.
Be direct with your expectations/rules. Bring your own condoms etc.I've never been to a gathering that's got out of hand and they've been a fun experience, either when participating or just watching.
Give a very general idea of your planned location (ie "within 2kms of ............ Road). By doing this you can be a bit flexible on your ideal spot.
Finally, let the punters know what to look out for (a red station-wagon, or a white van etc)
Hope these suggestions may be of some use.
You already have a good number of willing participants, judging by earlier responses.
