The marriage cannot proceed.
It is vital that an authorised celebrant is satisfied that both parties genuinely consent to the marriage. If at any point a celebrant is unsure of the genuine consent of either party, he or she should not proceed with solemnising the marriage.
An authorised celebrant might have been satisfied that a party was mentally capable of understanding the nature and effect of the marriage ceremony before the ceremony was due to be conducted, but might form a different view as a result of the party’s conduct during the marriage ceremony itself. In such a case the authorised celebrant should not proceed to solemnise the marriage until satisfied that the party is mentally capable of understanding the nature and effect of the marriage ceremony.
Other consent issues that arise on the day of the marriage ceremony can include, for example, duress or a party to the marriage who is drunk, intoxicated, under the influence of drugs, or otherwise appears to be in an altered mental state to an extent that this could impair their ability to consent to the marriage.
The same applies to the witnesses. A witness cannot sign if in an altered mental state to an extent that this could impair their ability to comprehend the ceremony.