I would be discussing your ideas with RMS before wasting too much time/effort on fabrication.
I have just had some mods done to my box trailer by an accredited trailer manufacturer where he advised that RMS (noting you are in NSW) are cracking down on "customised" trailers and requiring all sorts of "certification" of design and components before a trailer can be registered...
That said my next door neighbour made a great "fire" trailer by mounting a couple of old home heating oil containers onto a Morris 1100 rear subframe and associated suspension.... The subframe was joined to the towing socket by a couple of scaffolding poles. All for strictly off-road/non public road use
Cheers, Ian