Have not used the day/sleepers on the IP niknak but had first class similar grade in Europe overnight and recliners anywhere do not make for the best sleep anywhere unless you are dead tired and/or full of grog.
I ended up trying the floor in Europe first class one time much to the horror of Europhiles - another time in a compartment I had to myself, being able to stretch out was far better.
There's a piano bar, lounge bar and all that so maybe allnight partying is the go and there's not much in the way of scenery to be missed across the nullabor.
It is a long way to go in my book just to come back again within a month, and as good as there are areas over there, also plenty to be seen eastcoast way for a month/
Consider heading just as far as South Australia,
www.igawarta.com if you want a good insight into indigenous culture in Flinders Ranges.
Wine country of Clare and Barossa Valleys, and south on Fleurieu Peninsula and you can take a tour over to Kangeroo Island, see
www.waywardbus.com.au and also think
www.wildlifetours.com.au is another and you could also do the drive Adelaide to Melbourne that way too or hire a vehicle with
www.rentforless.com.au and places like Victor Harbour, Robe, Beachport (high salt content lake) Grampians, and the Great Ocean Road, ferry across to Mornington Peninsula and up to Melbourne that way is about a weeks worth.
Have a look at
www.underdownunder.com.au and Tasmania you could stay in a month alone if wanting to some treks, but just touring about, anything less than two weeks and not everywhere will be visited.
Melbourne to Sydney, there's a mix of Coastal and alpine country with some beaut locations, Phillip Island, Wilsons Promontary, Tarra Valley/Bulga Park to cut back across to Latrobe Valley coalfields and if you want to see some big man made holes, there are lookouts about, but Walhalla an old Gold mining village is a lovely near hidden gem before moving onto Lakes Entrance, 90 mile beach, and either turn at Cann River to take some of the route inland or continue on to Mallacoota Inlet, stay overnight at Gypsy Point Hotel where Kangeroos graze in the carpark or the Sea Horse Inn near Eden (nice spot for a drink/meal at least).
And then going up the east coast to Sydney, you could enjoy yourself for easily another week, there being some great beaches, national parks, historic villages, even a boat ride out of Narooma to swim with some seals if you like, horse riding in coastal rainforests, plenty of stuff to do.