I can see where you are coming from, but it would be $400 in flights for a family of 4 (If you fly at 11am on a Tuesday) and 90 mins flying (If you ignore the time taken to drive and check in to the airport) still makes it more of an undertaking.
I think people do drive if the park is worth it, Jamberoo and Gumbuya demonstrate that, and heck its an hours drive for most of Brisbane when heading to the GC parks.
At the very least, I'd think the Central coast area could support a water park, and the case may improve if the HSR to Newcastle is ever built.
As for a dry park, yeah with Sydney being so expensive, it would probably be out at Bradfield, but fortunately since Sydney has expanded westwards, its now closer to more people than Wonderland was when it closed.