I was born in Canada but became an Australian citizen in 1975.?
You dont need to apply for Canadian citizenship because you already have Canadian citizenship. Although voluntarily acquiring a foreign citizenship before 1977 caused loss of your Canadian citizenship, your Canadian citizenship was automatically restored by Bill C-37 in 2009. This restoration of citizenship was automatic and involuntary, and does not require you to do anything. You can simply apply for a Canadian passport now at any time using your Canadian birth certificate.