At PER it is now $2.80 using a Smartrider autoload or $3.50 cash fare. Must be the cheapest in Australia.
Offpeak, ADL is the cheapest, either with the Admet card or a mobile device, or debit/credit card, at $2.60.
MEL gone up to $5.70 on 01/01/26.
Maybe BNE takes the main one, for now, at $0.50 per trip, does that mean a change at Toombul to the city, from the airport by bus is $0.50, but then, on the change to the train, is that a transfer, or a new trip, so all up its $1.00.
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But then, for ease of use, for PER, esp if using VA or any other non QFi/QFd flights, the train under the control tower, to the city is the easiest.
BNE is cheap, but from the 590 drop off point at the former Toombul s/c, or Toombul int, you still have to walk a bit to Toombul station to get the cheap train to the city.