Since the Original Poster isn't (yet) a diver, it's probably worth clarifying the issue of decompression injuries, which he/she will learn more about in class. They're quite rare, and probably even more rare in well-run basic Open Water courses, but in the unfortunate case that one occurs, a major challenge is that they're expensive emergencies to treat. A basic trip to the hyperbaric chamber can easily cost into the high four figures (US$). Many facilities require reasonable proof that they'll be paid, and many general (non-diving specialty) insurance plans expect you to pay out-of-country costs up front, for which they'll reimburse you later. It can be an added complication if you're unconscious and someone you're with doesn't have a five figure credit card limit. But like I said, the chances of needing this is fairly rare, so it's probably not something to be worried about, but just to be prudently prepared.
On a completely unrelated note, I understand at least in some parts of Australia, you're required to have a dive medical performed by a certified local Australia dive medicine doctor. Wondering if this applies to OW certification in the Sydney area.