I spoke to my colleague at work (the consultant midwife) and she said if you arrive in Adelaide work ready (with APHRA reg) you shouldn't have a problem getting work. She said there is always a shortage of midwifes (particularly out in the country but being a new migrant I wouldn't necessarily advise you go there - there's a reason why they can't get people to work out there). I asked the specific question about being sponsored and offered a job before you come out here, she said it isn't a practise she has heard of happening in Adelaide.
However I would speak to the internal recruiters at the hospital and ask them directly, you should be able to find a contact on the SA health careers website. If you do end up going the independent skilled migrant route I would just make sure you have your APHRA registration before you get to oz.