When I lived in Sydney I billed out at around $50 per hour plus GST. In Adelaide, I thought I might have to go down a bit, but there is a reasonable amount of work and I ahve a few clients to fit around writing and other commitments, so it is anywhere between $40 - $80 depending on the service required. Basic bookkeeping only starts around $40 per hour.
Don't set your rate too low or you sell yourslef short. People will always want cheap, but then there are people who are happy to pay more for the right person.
Check the market in the area you are going to service to see if you are in the ball park as the other bookkeepers, but it really comes down to you and your service.
Remember to factor in any PI insurance premiums or professional memberships etc in your costs. If you just take an employee rate and add extra for annual leave, super etc, you will be underestimating your costs. You need to look at the other things too that you need as a bookkeeper in your own business.
Sorry for the huge post, I could just babble about this all night!
If you are looking for any other bookkeeping business tips, I have a free e-newsletter which I send to subscribers to help them start, run and grown their bookkeeping business. It includes marketing, getting clients, costing services etc. e-newsletter info