If you don't want plasterboard then 6mm villaboard will be your next choice, it's what they line bathrooms with. Being in Brisbane extended damp weather won't be a major issue, so should stand the test of time. Don't let it span more then 600mm between studs, though 450mm max span would be better. You can screw it to the steel if you want, though without seeing your exact set up, probably easier to screw timber battens in place first up and attach the villa to that. You could also line it in ply sheeting also, my SIL got a heap of 16mm packing sheets to do their shed, really just depends what look you want to achieve .