This is one of the entrances to a large area of rugged bush country, mainly used by hunters. Through the maze of trails here, you could find your way all the way down to the Whanganui River, and probably all the way across to Tongariro, although I wouldn't necessarily suggest attempting that if you didn't know what you were doing.
For those less bush inclined, there is this simple park and grab cache at the entrance. If you drive in a few metres, there is a large area you can park your vehicle and walk back out to the cache. Be careful obviously as this is a state highway, although you don't need to go near the roadway itself to find the cache.
BYO pen and happy caching!