We have it through our PHI, but there are some conditions on it... Something about only for emergencies... So we decided it was better to be 100% sure that we were covered no matter what the circumstances and have also taken it out with Ambulance Victoria. We just got our renewal and it was only $75 for the year, and that's for a family.
I was told something similar a few years ago when I had to go to hospital. My Mum had driven me because I didn't have ambulance cover. The nurse at the hospital told us that if I had told the paramedics I felt like I was having trouble breathing, then that would have made it covered under most PHI's. Secondhand info though, so can't comment on how accurate that might be.