If you get married overseas it may be seen as a legal marriage here in Australia—as long as you have a marriage certificate from the registering authority from the country you were married in.
Your marriage does not get registered in Australia as it happened overseas and is already registered.
If you get married in another state or country, your marriage needs to be registered with the interstate or overseas registry.
The wedding certificate given to you by your registered marriage celebrant on the day of your marriage ceremony is not an official marriage certificate and cannot be used to prove your marriage to government agencies or banks.
You do not automatically receive a marriage certificate after getting married. You need to complete the marriage certificate application form and pay a fee.
Of course, your marriage has to meet the current definition of marriage in Australia so it must be between a man and a woman (and you can’t already be married to someone else).