The Box Vale Circuit is a lesser travelled walk in the Southern Highlands near Mittagong. It follows an old mining railway cutting at first, with valley views on a short side trip. It then descends very steeply into wilderness, follows a river, then climbs back out, passing the attractive Forty Foot Falls on the way. You may well meet other walkers, but probably not in the middle half of the walk, which skirts a section of the Nattai River and feels a bit more remote. Track notes at the end.
This walk is in the useful guidebook for Sydney walks, A Day in the Bush, which describes a circuit walking clockwise. This link to Google Maps shows the start on Box Vale Road. A search online will reveal plenty of descriptions of the first part of the walk to the end of the railway line, and to a lookout via a short side trip. To continue on you have to descend very steeply with the aid of a cable down to and across the Nattai River, then walk along the river, crossing it again once, and ascending back out of the valley near Nattai Creek. You’ll soon come across Forty Foot Falls, and then continue to ascend back to the railway route, taking a left to return to the start. We’ve done this walk a few times and in wet weather there were quite a few leeches along the Nattai River, so beware.