Yeah my post office where I live (small town) is open on Saturdays, but there is a sign up inside behind the counter saying:

Please Note: Mail Pick Up Times: Monday - Friday 5pm, ONLY! 

When I use to live in Melbourne (2 years ago) I was also informed when I asked about mail being posted on Saturday that it wasn't picked up til Monday morning. I think it differs depending on location.

I know most post office open Saturday for people to pay bills, as we have all read in the newspapers and on the net, how Australia Post is struggling with letters and parcels(which I find strange since more Aussies are shopping online), and making their money through being agents for banks and for utility companies.