This map shows the most straightforward tramping route to Ranger Biv.
Head north from the Casey and cross the Poulter to Fenwick Stream. Travel up the broad open gravel bed of Fenwick until it gorges at the side of the hill. A cairn marks the point where you take to the bush. The spur high on the true right of Fenwick Stream has excellent sections of deer trail. Honora and I have clarified the route linking these sections of deer trail so it's fairly straightforward to follow them all the way up to the biv.
Higher up the route goes through a stand of young beech pole forest. The line marked on the map is easy enough to get through and almost entirely avoids the windfalls.
Any place where it's not obvious the line of the route is indicated with plastic ties.
Get up there.