Marchwood Park is owned by the Motueka A & P Association which has been running annual shows since 1935. At the show you would once have seen lots of sheep and cattle but there are not nearly as many these days, a reflection of the changing rural scene. There are always lots of horses though. I don't think the feature at GZ was ever used on show day, being built for another type of event. My thoughts are that it's not for the faint-hearted - - - you might think differently.
The park is now a popular camping area, though if you have thoughts of using it you'd best read up on the rules first.
If you're entering the park on foot, the best access is from Marchwood Park Road but you can drive closer if you enter from College Street. The signs don't look at all welcoming but hey I'm sure you can think of a really good reason for visiting. If you get it right you won't need to clamber over any fences.
PLEASE READ THE VERY SPECIFIC HINT AND IF YOU STILL CAN'T FIND THE CACHE, READ THE HINT AGAIN AND TAKE A VERY CAREFUL LOOK AT THE MIDDLE POST AGAIN. THERE IS ONLY ONE MIDDLE POST!!!! SEARCH IT FROM THE VERY TOP TO THE VERY BOTTOM.