The EU can (and in fact does) mandate that you comply with the GDPR. It cannot, however, enforce compliance since there is limited legal leverage. The Australian government is unlikely to help the EU to bring a case unless there'd be a treaty or an agreement (Safe Haven laws in the US for example). They could theoretically go via your revenue stream and seize your European customers payments or hold you or any officer of your company liable if you ever set foot on European soil or hold any assets that your company has in or moves via Europe. That's all very unlikely to happen over minor infractions, but some business folks already had their private jets impounded for outstanding payments as for example the Thai Prince learned the hard way: http://www.airliners.de/kronprinzen-boeing-in-muenchen-besch... (sorry, german only, but google translate should correctly translate the gist of the story)

All of this is nothing new, it's been working like that for centuries, back when business correspondence was still on old-fashioned paper.

As long as you don't go on holiday to Rome, you'll be fine.