Ann: Western Sydney Airport Notice of Intention, page-52

What about Tokyo? They have Haneda and Narita. Both are international but one focuses more on tourist visitors while Haneda picks up the premium business travelers.

Or what about Shanghai with Pudong and Hongqiao? Both cater for domestic and international flights.

Or New York with JFK, Newark and La Guardia?

All these cities have multiple airports serving both international and domestic flights. What these cities have in common is difficult ground transport and so you pick the airport closest to where you live, or where you are going.