Check the train travel times (CountryLink website) - 4 hours 20 minutes Canberra Station to Sydney Central, not counting the 20 minutes it takes me to drive to the station here.
Unless I break the 110km/h speed limit it takes about 3 hours to get to the western suburbs. So maybe 3.5 hours to the Sydney CBD?
And of course following my suggestion Sydneysiders can still drive or ride to Canberra in the apparently short time they think it is, while the Wagga Wagga, Coome etc. folk will still be happy.