I'd start here:
http://www.careforkids.com.au/search/lo ... enu=searchI would say it would range from $50 a day at the cheapest up to $120 or so at a guess. It really depends on the location. If it's right in dowtown Sydney expect the most expensive, if it's farther out in the suburbs it would be a lot cheaper.
I know our childcare centre in Brisbane does give a bit of a discount for multiple days and children but it's not a huge difference. Also it depends on what visa you are coming here on. If you are on a temporary visa you wouldn't be eligible for anything.
If you're on a permanent resident visa (or citizen) then you can get benefits from Centrelink. They will pay for a percentage of the fees using a sliding scale based on your income. If you make over a certain amount you would get nothing, if you make very little you'd get a lot. It also depends on if you're a single parent or if you've got a partner and you're both working, etc.
So I hope that helps a little. Unfortunately without knowing what visa you're on it's hard to answer. Your best bet is to figure out roughly what area you're in and call or email a few centres to find out. If you are on a permanent visa you can get an idea of benefits here:
http://www.centrelink.gov.au/internet/i ... imator.htm