I have always found the best deals by using the moneysavingexpert website. One of the tools it has is called the Travel Money Maximiser, which can be found at this link
http://travelmoney.moneysavingexpert.com/
The rates for this are updated daily and provide you with a wide range of suppliers. You will also find that the rate can vary according to how much you wish to change. This takes this into account as well.
It is also worth reading other tips from this website about overseas travel, particularly about which are the best credit/debit cards to use overseas etc. I find it a very informative site. Hope you do as well.
For example, best rate shown on there today is £1 provides 2.572 Australian Dollars.
Have a good trip Jimmy.