This is the entrance to the Pear Orchard picnic site (as described in our Pear Orchard View cache), and it's the entrance to walk to the top of Battery Creek waterfall and the Battery Creek U-Turn cache.
Despite the gate normally being open, there are formalities to access this area. For Pear Orchard picnic site, there's a phone number on the info board, and permits to go to the waterfall can be obtained in the village. Permits for the waterfall are minimal cost and are mainly to keep a record of who's been given permission to go onto the plantation property. Waterfall permits also allow you to go down into the gorge to the bottom of the waterfall - which is amazing and there's a cache down below. There's a way down into the gorge about 100m to the right of the top of the falls, but it's very steep. It's easier to get into the gorge from the other side - access via the gate near the cemetary.
This is also the parking to walk to Adam's Calendar which is through the gate across the road about 4 kms away. There are signs warning you not to enter any caves or mineshafts. Take the warnings seriously - all shafts and caves in this area are unsafe.
The cache you're looking for is - well, a cache, and it's hidden somewhere.