Your legal responsibilities are the same whether you marry a Canadian citizen or an Australian citizen. What matters is where you marry. If you are living in Canada, the responsibilities may be different than if you are living in Australia. (This website is not run by a Canadian lawyer, so we cannot comment on family law.)
If you are asking about your responsibilities if you sponsor your Australian boyfriend for permanent residence after he is your husband, that is a different question. If you sponsor him, you will have to pledge that you will repay the government if your future spouse ever needs social assistance.