Looks like I'm not the only one advocating buying your petrol on a Tuesday in Sydney.
Here's a recent article from The Age, where the ACCC† boss says basically the same thing I did in my post. He goes on to say that there is no "obvious evidence" of collusion, despite the fact that there certainly seems to something going on.
Anyway, I went to buy petrol last night, and the line of cars waiting went out onto the road, so it looks like people are catching on. Not sure what that means, though... perhaps they'll just change which day the "cheap day" is to catch everybody out.
† That's the Australian Competition and Consumer Commission, Australian's consumer watchdog.