If by 'pretty much proven', you mean 'there is vague evidence, a significant part of which is based on native paintings of what might be Portuguese ships.'
And given some of the things painted on rocks by Australian natives, its hardly surprising that something which looks like a European ship would appear.

Regardless, the first CONFIRMED and undisputed exploration of Australia was made by the Dutch.

As to the rest, Magellan missed Australia by quite a ways. He did manage to circumnavigate the world, but I'm fairly sure it had been confirmed some time ago that the world was round (even in Europe), and the 'flat-earth' theory was generally confined to more rural areas.