Correct me if I'm wrong, but I believe that there are speed restrictions on the existing track between Joppa Junction and Canberra because of its relatively sub-standard condition. As RTT and simstrain have already intimated, it would only require a modest investment in upgrading the track to significantly improve journey times.
The upgrading of the Main South from Campbelltown to Goulburn is a different matter and electrification in combination with some critical curve easing could play a significant role in reducing the journey time even further to make it more competitive with car and coach travel. The steep gradients are not such an issue with electrification or even modern diesel freight locomotives. It's the curvature that slows down journey times. Electrification will boost the effectiveness of the new hybrid regional fleet.
If the journey time between Sydney and Canberra can be reduced to below 3 hours, then it would significantly improve patronage and frequency. A frequency of a service in both directions every 2 hours between 6:00 am and 8:00 pm should be goal.
The current trip time from Goulburn is 1h 36min for 106 km, slow by any measure.
- the 57km section to Targo isn't too bad averaging 127km/h (removing 1min for station stop)
- 11km to Bungendore is painfully slow at 29km/h, yes I've checked this multiple times, but I think there is a STAFF change at either station which obviously can be improved
- 28km to Quean'ie at 62km/h and this is through the windy valleys
- 9km to CBR is 39km/h
So if we make some track improvements
- 57km to Targo, small increase to 140 km/h
- 11km to Bun, increase to 95 km/h
- 28km to Quean, increase to 100 km/h
- 9km to CBR, increase to 70km/h
Still allowing 5min for the 3 intermediate station stops, we are down to 55min, saving 40min
So the trip to Sydney is now down to 3h 30min, without factoring in any benefit north of Goulburn with the new trains, but I think any benefit would likely be off-set south of Goulburn compared to above as I may be a bit to optimistic.
I've said it before, if the money was available, I think its better to head west from Bungendore direct to Canberra Airport (terminus), huge saving in time, but yes there is some more serious real-estate to traverse. But with hybrid trains and only one track, you just bore a single tunnel that also wired and save likely another 10-15min as the new track and tunnel would be rated at 150 - 160 km/h and you save about 20-30km in distance.
EDIT, with this run time, then you could have a minimum 2h frequency Ex - CBR passing at Goulburn - Joppa JCT on the dual track. No need to have complex/costly passing on the CBR branch apart from dealing with existing freight movements.