It's possible that this inflated figure was to be used to rule out Badgery's Creek, the site planned since 1984 with a forecast of 25 million passengers per year underpinning it. So let's see how good the new number is. I know he's full of crap. The 60 million figure comes from a simple model of exponential growth.
Exponential growth works just like interest growing on a loan with no payments.
It's a fair fit to the historical record. Also shown in the graph are some alternative models. The magenta line labelled 1997 Logistic shows a mathematical model of logistic growth, using just the data points available in early 1997. While there's not much difference in the historical fit, the picture soon changes a little when you project into the future. This is like expecting VCR, car sales or mobile phone sales to grow without end.