Well, thank you gentlemen, not only for the fast replies but also for kicking my brain into gear.
Didn't even think about axle width. Went out and had a good look and would definitely need a new axle to give tyre clearance if shifting the wall.
Now, if that was first gear, time to change gear. Who mentioned DIY... I'm pretty handy around the tools, got some mates who can weld and owe me favours for free work I have done for them including one whose old man owns a steel manufacturing factory, so maybe a few beers and a bottle of scotch might ensure getting genuine prices on materials and assistance. Probably should have asked him first but wasn't thinking along those lines.
2nd gear... a lot more research including problems associated with upgrading a registered trailer or even building one from scratch. VicRoads can be a right royal pain at times but other times go out of the way to help. Pot Luck. The good thing is I have time on my side. This doesn't need to happen immediately.
Also a special thank you to Dazzler. I have found your article - Quality Budget Priced Trailer - http://www.myswag.org/index.php?topic=36094.msg578367;topicseen#msg578367