Mondo
Yeah what JC said. In QLD go to the Dept road Transport they will give you the plate and VIN number ( which will be registered to you) after you give them $$. You can have plate engraved or stamped. You can tow your trailer to the weigh bridge then to roadworthy then to DRT with out it being registered. some where between roadworthy and DRT you will have to stamp/ engrave plate and mount to trailer before DRT inspect it. I beleive that this is for braked trailers (roadworthy inspection)
You dont have to weld form the VIN no onto trailer i think its only for security purposes. There is a metal recycling place near simms metal on Spillar Ave, Outer Harbour MKY that have a weigh bridge which you can use for a gold coin donation.
Regards Angus