Hey Reever, Just put a box on the back of mine that sticks out about the same (40cm). I'm in QLD and rang the transport mob and after being passed from clown to numbscull i finally got someone who seemed to know what they were talking about.
Their advice was that it is fine as long as the blinkers/stop/tail lights can be seen on the 45 degree from a "safe travelling distance". They were unable to say what that distance was as if you were doing 100k's down the freeway it would be a much greater distance than if you were pulled up behind someone at the traffic lights. (so basically, I just stood at the rear of the trailer to one side, took 4 good paces backwards and as long as i could see the lights on the other side of the trailer i was happy).
You must also be able to see the number plate at all angles from the furthest most projection. (therefore, best place for the plate would be on the tool box itself).
I wanted to get the longest box i could on the back so opted for one that already has the sides of the box cut on the 45 degree and this enabled me to have a box anout 1400mm long. If you are just going for the conventional rectangle box, you may need to go a little bit shorter so as you can still see your lights on the 45.
Hope this makes sense and just remember, this is based on QLD advice.
Cheers.