In my opinion there is no reason why fares should be any different for the airport line. The airport line should be just integrated into the rest of the network. It is just a suburban train to the airport.
The fare increase on an airport line is considered a surcharge fee. The Reserve Bank of Australia are in the process of banning this practice. Ticketmaster, telstra, airlines, banks, taxis are being forced to remove these charges..
Also charging a premium to use the airport line is not really an incentive for passengers to use it. It defeats the purpose of building the rail-line if it is cheaper to use a bus, taxi or car to travel to the airport. The idea is to make public transport accessible and attractive for users. To take vehicles off the roads.
Yet Sydney had a premium on the airport and used standard rollingstock. And airport passenger faced full trains as well. Brisbane was the same re the surcharge, but at least it was the destination, and they often did use medium distance trains on the line with toilet and luggage space, thought this has changed with normal sets used a lot more.
They think they'll make more money to use the airport line, but in reality would make more if it was cost effective !
Anyhow it's all just pipedreaming for now. I believe the airport line is not on the horizon for Labor, so it is now all academic.