"I had a similar issue last week. IFR from Canberra to Melbourne. Had planned sbg to YBSS."
If you had been allowed to fly this route you would have been like the bloke driving along the Monash freeway - on the wrong side of the road.
I believe that if you wish to plan on a non-published route, in CTA, you are supposed to obtain prior approval from ATC. If you get that approval then you should expect to fly the route. If the route is not approved there will be an alternative suggestion.