RAAF Derrick Trucks

[Allan Currey]

Here's some more to add. As you can see in the documentation, it is definitely called a derrick truck. Issued to engineers and the like, it would have been used, among other things, to load/unload the semi trailers that bridging platoons used to carry pontoons, bailey bridge sections, etc.
As for slewing control, there are 2 attachment points just behind the the tip of the boom. I think that simple ropes were tied here and used to pull the crane around.

Cheers,
Allan