If you just want to use ancestry.com.au to search and download documents I found its free to use at my local library (Balwyn VIC) and I believe its free in all library's. I was able to get a library card on the spot with a valid drivers licence - even thought the address on the licence was not in my local council area. Then use their computers to search and download documents to a USB drive.
Normally you have to give them a credit card to get 14 days free access and then its $24.95 a month
Here's a link to whats available in WA
http://www.slwa.wa.gov.au/find/guides/family_history/Family_…
And here's a description of the library edition
http://www.nswnet.net/e-resources/ancestry-library-edition
My my case the library had a bunch of links to family history sites on their web site but no mention of ancestry.com so I guess the message is if in doubt - try it.
Nice! thanks.