The website mymotormouth used to do the monitoring but now they seem to have stopped.
The ACCC also does this and you can find their charts here.
Here's the day's for the last week
City Cheapest day (week ending 1/5/2011)
Sydney Wednesday
Melbourne Wednesday
Brisbane Monday
Adelaide Tuesday
Perth Wednesday
For Melbournians, here was our graphing:

From Jan 2011 to now, we've moved through Mon -> Fri. It was Fri for awhile and if you don't monitor, you end up paying at the peak for NOW Fri is the most expensive.
For all other states, you can find your own graphings here:
Brisbane
Sydney
Adelaide
Perth
Good luck beating the bowser people!