BIRTH CERTIFICATES
The Celebrant is required to sight evidence of your birth date and birth place for both of you. This can be either your original Birth Certificate or an official Extract of your Birth Certificate.
If neither of those is available a Statutory Declaration by you or your parent can be submitted. 
"A statutory declaration made by the party or a parent of the party stating that, for reasons specified in the declaration, it is impracticable to obtain such a certificate or extract and stating, to the best of the declarant’s knowledge and belief and as accurately as the declarant has been able to ascertain, when and where the party was born" (Marriage Act 1961 42 1b(ii).
A passport issued by a government of an overseas country showing your date and place of birth can also be used.