This is an excellent walk, and one I would rate as family friendly. The car park is around 810 metres elevation, and the base of the falls are around 915 metres elevation, so it is a relatively flat walk. I took a two wheel drive vehicle. The drive in is on rough gravel roads as per Tastrekker's post. Heplittledoggie PM'd me these instructions: Mersey Forest Road, to the Dublin Road junction, about 2 km south of the Rowallan Dam, travel east on the Dublin Road to its junction with Little Fisher Road, turn right into Little Fisher Road and travel for 6 k's or so past the bridge over the Little Fisher River, turn right and then turn right again, past a sign for the Walls of Jerusalem National Park, continue until the car park. This proved to be spot on. The track narrows more and more as you head in, and you're left hoping that there will be somewhere you can turn around, otherwise you will have some fancy reversing to do. There is, the car park at the end.
The walk starts from the carpark across a little foot bridge.
You then walk about 650 metres to the river, and cross on an old log bridge.
Across the bridge the track continues another 100 metres to an intersection and you will see a Cairn on the left fork. This old vehicle track continues on for another Kilometre. A short way up the track from the corner you will cross an old bridge over a stream.
Once Nik crossed I had but one focus, get across before I had too much time to think "hey is this really a good idea?".