I just discovered a page on the ACCC's website, where the graph the average price of petrol across Sydney on a daily basis.
The page confirms something I've known empirically for a while now: the best day to buy petrol in Sydney is Tuesday. The only people who should ever buy petrol on any other day of the week are taxis or other people who use more than a full tank per week.
You can see in the graph above that the troughs in price (and a trough is usually at least 10c per litre lower than a peak!) usually occur on a Tuesday, just before the peak on a Wednesday or Thursday.
So spread the word: buy your petrol on a Tuesday! Actually, better advice would be to monitor this ACCC website and always time your petrol-buying to coincide with a trough... After all, the cycle does move around a little bit. Six to twelve months ago, I remember the trough usually occured on a Wednesday, but it's been slowly moving backwards in the weeks.