No idea why but VicRoads have a whole section on roll cages and they require Engineering. Like any other mod, you don't have to have it engineered but that comes down to how paranoid you are about insurance companies, etc.
Given that there are plenty on here with tyres way larger than what is legal without engineering, there's a lot that are happy to just do it. It's all up to the individual I suppose on what they are comfortable with.
I'd be happy to build a nice strong roof rack as suggested - that seems to get around it anyway. :)

