Copied from:
http://www.smartraveller.gov.au/tips/dualnat.html#4
Entering and leaving Australia
To avoid delays at airline check-in and again at the border entry point, Australian citizens who are dual nationals are advised to leave and return to Australia on their Australian passport. An Australian citizen cannot be granted a visa for Australia.
When entering Australia, all Australians, including those who hold dual nationality, must be able to prove that they are an Australian citizen. An Australian passport is conclusive evidence of a person's identity and citizenship, and provides the holder with unfettered right of entry to Australia.
An Australian citizen who arrives without an Australian passport may be delayed until their identity and claims to enter Australia have been checked. If a person holding a foreign passport claims to be an Australian citizen, immigration officers must confirm and verify this through official databases. This will invariably cause some delays to the person while the authorities undertake the necessary enquiries.
International airlines have an obligation to ensure that they only carry appropriately documented passengers to Australia. In the absence of an Australian passport, airlines are unable to assess an Australian citizen's claimed citizenship at the time of check-in and may decline to carry the traveller. Or, they may have to make inquiries with the Department of Immigration and Multicultural and Indigenous Affairs in Australia to seek approval to carry the passenger, which takes time and may cause inconvenience to the passenger.
I am aware that this might not assist the OP concerning dual citizenship with AUS and CAN, but there might be some similar Canadian Govt site that could assist in the same way.