If its in a few years time, you would probably get in no worries. If you put your name on the lists now, you could always decide down the track. I think it was something like a $50 fee for going on the wait list for Mt B. I think Mt barker only go up to yr 10 and that has only been recently I think. Not sure what ages you are looking at, but if you want them to go to a waldorf high school I would probably head for a school that does include it. In the hills there are only a few options for high school and they aren't great. I didn't want to send them to a waldorf primary school and a mainstream high school as they are just so different. I didn't think it was fair.
I don't think you will have any trouble getting approval. Basically, if you register you get a visit from the education dept once a year. The woman who does it has a farm up the road from us and she is just lovely. She homeschooled her own kids for a while. I haven't registered yet, but according to those who have, she is very good at translating your stuff into edubabble to take back to the dept. She is very supportive of homeschooling and also of peoples choices and methods. SA doesn't require a lot in terms of evidence or record keeping. They basically just want to see an appropriate learning environment, chat to you about your goals and then some kind of proof you are doing something. So whether you show her some workbooks, a blog, photos etc. I think some people do a one page overview. They don't want very much, unlike some other states.
Your kids probably would love the steiner approach, I think mine would respond very well to it too. There were a few things that I found negative, but I don't know that individually they were deal breakers. Just that the combination of them and the cost was the clincher. I couldn't justify paying so much money for it, especially when I am planning to be working from home and could easily incorporate the same things into our lives. We also want about 4 kids so paying that much for schooling and knowing it will increase, we just couldn't do it.....
There are some independant schools in SA too. This one in particular springs to mind. http://www.kirinari.sa.edu.au/
They are a little school and also run homeschool workshops. They offer 1 or 2 days a week where hs kids can go for the day. When kids go is flexible. So you have the option of part time homeschool if you want, full time school or full time homeschool. There is a pretty big hs community though, so you will find there are groups everywhere no matter where you live. I also believe down at Yankalilla they also run hs workshops through the school. That one is something like you register and you are required to go to 7 or 10 days throughout the term or year or something like that. You can either go to the workshops or justy go to the camp at the end. Parents can also be part of the day so its more of a family learning environment. given you are looking a few years down the track, I would just keep your options open. Put your name down at the schools and by that time you will have a feel for what you are after. it will work out! xxx