If thats the case then SIA would need to offer the same fare as SYD-SIA as CBR people will drive to SYD if it saves them money or catch a Murray's coach to the Intl terminal. They might as well announce SIA-NTL if its just about postcodes.
I hope you are not serious with this statement:
If SQ wanted somewhere to fly the plane for the day, Canberra-Hobart might have been a better tag leg, I'm sure the Tassie govt / Hobart airport would have welcomed them with open arms & a bit of cash.