It has to be taken as fantasy
I first saw this movie when it was released in Australia (yes, I'plot m old) and have seen it few times since then. One thing has always stood out for me - the silliness of taking kids on a picnic from Sydney and ending up in 'the outback' not long after with a non-refrigerated picnic meal still nice and fresh.
It's just ludicrous.
Any Australian will tell you just how long it would take to drive a 1960s VW from Sydney to a location like those shown in the film. You'd need an overnight stop, or two. And much more than half a tank of petrol and a jerry can with some more, just in case.
I take this movie as a fantasy, it's the only way to deal with the one huge, screaming flaw in it. The movie itself is magical and fascinating and should be seen as an Australian classic.