Taken from the WA Dept Transport website:
Modify a vehicle
...Modifications that don't require approval
Many minor vehicle modifications can be carried out without specific approval, as long as they comply with the applicable legislation. Modifications that don’t require approval include:
Additional lighting 
Air conditioners 
Air shock absorbers 
Alarm systems 
Badge bars 
Body markings and speed striping 
Gearbox (pre 1976) 
Mesh stone shields for wind screens and lamps 
Mud spats and pebble guards 
Optional manufacturer seating 
Radio and stereo systems 
Rear-view mirrors 
Roof racks 
Seat belts for pre 1969 cars (no full harness types) 
Single tone air horns 
Stabiliser bars 
Sunvisors 
Torque rods and traction bars (not semi-tramp types) 
Tyre size and aspect ratio 
Venetian blinds and other internal screening systems are allowed, subject to a clear view to the rear...
Seems OK to me to fit seatbelts in WA without engineer or other approval...
(Sorry for the thread highjack mfc).
Cheers,
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