Some comply with ADR (australian design rules) and are permitted for use on the road, and some have only FIA (or competition use only) on them.
While the ones with an international or FIA only approval are safer than the australian standard, if they dont have australian approval, they wont be allowed.
Quite a few of them come with, or have available brackets so you can bolt them stright onto the runners in your car. Makes getting them enginerred quite stright forward.