Weddo,
You were on the money. I had a call yesterday from WA DPI and was advised (the confirmation letter is in the mail) that seatbelts and washers do NOT need to be fitted to my wagon, even though it has undergone some extensive modifications (such as v6, brakes etc etc).
This is what I was expecting given, as you pointed out, the "National Code of Practice for Light Vehicle Construction and Modification" (viewable at
http://www.infrastructure.gov.au/roads/vehicle_regulation/bulletin/vsb_ncop.aspx) that covers what you can and cant do in relation to all types of vehicles.
Cheers
Mike