As far as I know the morphology scan at 18-20 weeks is a 'compulsory' (not really, but they like everyone to have it I think) scan and is therefore goes thru Medicare. The other scans, dating scans and the NC one at 12 weeks are optional and while you get some money back from Medicare there is an out of pocket charge.
I have never had to pay for an US but have only ever had the 20 week one
HTH