We recommend parking at the North Road Lookout a bit further south and following the track down to the river from there. The walk is about 3km return, very enjoyable and far less steep than the direct approach; the terrain rating is based on this route. Alternatively, there is a track down from near the end of Edmonds Court. It is somewhat steeper, but about half the distance to walk. Although you can park less than 200m from the cache, getting to it from there is another matter. If you do choose to scramble down from Sterling Drive, please note that the slope is quite steep and you should add at least another 1½ stars to the terrain rating.
If you want to walk even further, the riverside track can be accessed from several places in Avondale Heights downstream, and from Brimbank Park upstream, where there is another cache to be found.
Once on the Maribyrnong River Trail, the best access to the hillside is about 50m south of GZ. You are looking for a small Sistema. As usual, please ensure the cache is well hidden before you leave, using the same camo as where you found it.
Have you visited the Geocaching Australia Forum?