Just had a quick play on Google Maps for the Victorians; it looks like staying inland (Hume to Gundagai and then across to Dunedoo and then Tamworth and Armidale into Coffs) would add 45mins (bear in mind Waterfall from Armidale to Bellingen has a load of roadworks at the moment so that skews the displayed time) over going via Sydney - it might be worth the 45mins in case Sydney is in gridlock hell.
We've done Armidale to Melbourne in 15hrs including breaks for the humans and the dog - Coffs is 2.5hrs ish from us.
Also dodges all the Sydney Toll roads....