Congratulations Seymore!
First of all, don't panic - it will be fine, and will def. not cost you the earth!
I got pregnant with DS2 about 2 months after we arrived here. I had to go through the GP to get my referral to the hospital, and had to pay for that appointment. You will also have to pay for any scans other than the 16 week anomaly one (unless you have reasons that you have to have a 12 week one - and they are pretty strict). My scans cost $120 each time (I had a few because of early bleeding), and as far as I can remember, you can claim back from some of it from medicare.
I had 3 options with going with the public system (I had DS2 at Royal North Shore) The first was shared care (you do some appointments at the hospital and some at the GP - my friend did this route because she didn't want to have to travel to the hospital each time), the second was midwife care at the hospital, and the third was group midwife care. I ended up going with the third - it's pretty popular, so you would have to get yourself to your GP asap and then phone the hospital to see if there are any places left - it meant I got a midwife allocated to me all through my pregnancy, and she was the one who also delivered DS2. I cannot praise the care I received highly enough - it was brilliant. I ended up with several complications in my pregnancy, and the consultants were also lovely. And the wards in the hospital only had two beds per room - bonus!
You won't see a midwife until 14-16 weeks (I found that quite strange) so any problems up to that point would have to go through your GP. Your GP does seem expensive - ours charges $60 and we get about $40 back I think. There are a few bulk billing GPs around our part of Sydney, but they are few and far between, and you often (unsurprisingly) have a longer wait for an appointment.
In terms of getting baby equipment, I spent a lot of time browsing ebay and gumtree (it's a shame it wasn't a few weeks ago - you could have had our cot - as it was it sold for $25 on ebay, so there are plenty of bargains out there!)
Sorry if I've repeated any advice from others - I haven't read all the replies - am packing to go back to the UK tomorrow (eek!) Feel free to pm me with any questions you have!