I vehemently disagree. You'd be hard pressed to find similar jobs across Australia with identical pay and conditions.
Different drivers may be working charter (hats off to them, it's a tricky gig!), school buses, route buses or some combination thereof.
Different depots have differences in quality and quantity of passengers.
Between different operators there are differences in policies and procedures. E.g. at some a driver will sweep (sometimes mop) and fuel their buses, at others there are refuellers and cleaners.
Then of course you have the physical workforce. Some drivers are better than others at their job; they're more diligent with reporting defects and onroad issues, they care about the passengers, and endeavour to be polite.
With all these nuances in mind, there's no reason, by my thinking, why all operators should pay the same. If you want good staff, you'll pay them well and look after them (but not when they're blatantly engaged in misconduct). An operator more concerned about the bottom line may well pay award rates and churn through drivers. It's not for me to say who is worth more, to be honest, though.