Awww, too bad. Still, it sounds like you have a good plan. My pre-med adviser told me that no one she's ever advised has ever regretted taking a gap year. 😀
I didn't have any experience with the medical system in Australia. My Uni didn't have good information on volunteering/medical internships, probably because the medical system there is so different. As I understand, there are not really separate undergrad/medical programs In Australia. Sydney Uni had a separate campus for health professions, which is quite far from the main campus. I'm sure you could manage something, but I'm not sure that it would look particularly good with adcoms. I don't think it would look bad--it's just that the medical system is very different, and it wouldn't be considered volunteer work, so I doubt it would help. You might want to travel to Australia *just* for travel, and do a medical internship elsewhere in your gap year.