Possibly some sort of entrance to the spillway near the top of the construction?

View from the bush area on the non-carpark side of the dam

Closer look at the upper areas of the spillway from the non-carpark side of the dam

The lower parts of the construction, on the "dry" side of the wall, as seen from the non-carpark side of the dam

Wider shot of the lower parts of the construction, on the "dry" side of the wall, as seen from the non-carpark side of the dam

So ACTEW Corporation haven't contracted out the construction of the spillway?

Another crane near the wall, although the real reason I took the photo was that it was the only view of the water that I could get from this area of the bush

Further around, with camera poking through the wire fence. The Dam side of the spillway construction

Closer shot of the dam side of the spillway construction