During your wedding, the celebrant must say the following passage, called the Monitum;
"I am duly authorised by law to solemnise marriages according to law.
Before you are joined in marriage in my presence, and in the presence of these witnesses, I am to remind you of the solemn and binding nature of the relationship into which you are now about to enter.
Marriage, according to law in Australia, is the union of a man and a woman, to the exclusion of all others, voluntarily entered into for life."
This may be offensive to you or your guests, but the marriage will not be valid without it. A statement before or afterwards can be made to clarify why it is there.