Go all the way down to the end of Washbourn Road, to the parking spot at the end of the road (you don't need to go past the yellow "Farm Access" sign.) Walk along the Onekaka River as it reaches the sea, and then follow the beach along to the north. At high tide, the route to the cache might get blocked, as the tide can come right up to the clayey cliffs, but a low tide, this is a great beach!
When you get to the coordinates given, the cache is in an easily accessible place, off the ground.
No pen.