The cache is a Bison tube attached to the fence. At a high point with a view over the vinyards and model aircraft flying area. According to the sign the club days are Tuesday, Thursday and Sunday at 0830.
Access is via gravel roads along the river, the same track as http://www.geocaching.com/geocache/GC52JF9_twoamarina-and-the-river-marlborough#
You may get a bit dusty, but won't have any trouble in your car. Stealth required if the club is operating, you could use the photo opportunity excuse, but watch for low flying aircraft.