With the XPTs operated by NSW TrainLink, if travelling on a day service such as ST23 from Sydney down to Melbourne, insist on a forward facing seat in car A, the sleeping car that has nine compartments with three seats each when in dayime mode.
Often no one else will be in your compartment. You have to call NSW TrainLink as the booking engine doesn't allow selection of a seat, and most often it will book you in car B or one of the small number of first class seats in car C, which doubles as the takeaway buffet car. Call centre staff can change your seat.
Why not buy a NSWTrainLink first class Discovery Pass, as suggested above, valid for six months for A$550?
That way, you can travel to Brisbane (very scenic), and if you say leave on the first day train just after 0700, you can have lunch somewhere such as Wingham on the way, then travel a short distance on the lunchtime train ex Sydney and finally join the Brisbane-bound XPT further up the track. It has a very early arrival in Brisbane.
With Discovery Passes, if first class you can book a sleeper for A$88 one way, but there are only 18 berths on the XPTs and when I last travelled, it was only being booked at night to one per compartment (although couples could change that once on board). If back to two per compartment, I prefer a lower berth.
A bargain price that will get you by connecting road coach even to Coonamble, Bourke, Lightning Ridge with its bizarre opal mines, Adelaide twice a week from Broken Hill by road coach. A round trip is poissble: XPT to Melbourne, Journey Beyond's separate fare 'The Overland' twice a week to Adelaide, NSW TrainLink coach Adelaide to 'the Hill' and then NSW TrainLink XPlorer railcar (once weekly only - line currently affected badly by flooding) to Sydney. With the road coaches, you may need to input a different origin into the booking engine (i.e. 'Broken Hill Town' not just 'Broken Hiill', the latter being the railway station).
A very good trip is NSW XPlorer railcars 'Northern Tablelands Express' to Moree, then road coach across country all the way to Grafton City, connecting to the XPT back to Sydney. You will need to stay in a motel in Moree. However there may be Inland Rail works when you visit, perhaps still closing the line north of Narrabri. The 'NTE' has a small on train buffet and allows you to observe the huge procession of (thermal) coal trains between just out of Broadmeadow, NSW through Maitland and Singleton to Muswellbrook and Narrabri (empty northbound, loaded southbound). Newcastle NSW is the busiest coal port in the world and thermal coal is now even being sold to Europe far more than it used to be. The 'NTE' splits at Werris Creek with its mini rail museum - other section goes to scenic Armidale NSW via Tamworth, an alternative if the Moree section is partly shut. You would need to connect by coach to Glen Innes to then next day join the crosscountry coach across to Grafton City: