I think there is more Qantaslink Turbo prop sevices out of Canberra than there is Mainline Jet services.
So I don't think that wether it is a Jet or not, makes a difference.
I think that it comes down to the Government travel officers just couldn't be bothered looking for cheaper fares.
It would be interesting to read the policy on booking government travel.
Just looking a the schedules for the same day
Qantaslink fly CB to SY 23 times per day. 5 of them are mainline flights.
Rex has 6 flights on the same day.
If I was a politician I would prefer the flexibility of the carrier with the greatest frequency.