Was your iPhone 4 officially unlocked when you bought it in Canada? Canada normally sells unlocked iPhones which means that you pay full price and have no contract and you just buy a sim card for your phone. Did you do this when you first bought the phone?
If so, then yes, you should just be able to buy an Australian sim card and insert it into your phone and then use your phone. If the phone is locked to Telus in Canada and you have a contract with Telus in Canada, then you will have to contact Telus in Canada and ask them if they will unlock your phone for you before you can insert an Australian sim card into it.
If the phone is locked and to get it unlocked, contact Telus - they will be the only ones who can help you.