From memory as the GVM of the vehicle increases the registration cost increase. So reduced GVM may have meant the vehicle fell into different pricing bracket for registration and possible insurance. The powers that be may have deemed that over 500kg payload may have deemed the vehicle a commercial vehicle at the time. I remember getting a shock when I went to rego an old Gemini panel van I had years ago. The cost of rego and third party in NSW where higher for the Gemini van than the VB Commodore I had. When I enquired why with the insurance company I was told the van was deemed a commercial vehicle even thou policy was for private use.