I don't believe SLS is the issue or required by Qld Transport, but a modification plate definitely is.
Without a mod plate, the compliance plate will say 5, and you'd have 7 seats. This can leave you open to fines/defect notice, and would void
any insurance
ouch.
Wouldn't be a big deal to do this though.
Try British Off Road or Gold City Land Rover spares, they both normally have a stack of D2 wreckers and bits