@timrichards a) partly held up by the private airport owners insisting that the government pay for the station be built underground at twice the cost, b) partly delayed by the government having to find a rail alignment that isn't going to result in the airport rail link more or less instantly going into administration (I'm looking at you, Brisbane) because the construction cost results in a link that is uneconomic from day one, c) price and time to CBD is competitive with Brisbane and Sydney.
But yes, a rail connection would be nice a decade ago.