Found this on www.smartraveller.com.au

When you are travelling with children, local authorities, including customs and immigration officers, may ask you to produce documents to prove that you are the lawful parent or guardian of a child travelling with you. To avoid running into difficulties make sure you always carry proper identification for yourself and your children, including any documents that might be required by the authorities of the country you intend to visit and by Australian authorities on your return.

When travelling outside of Australia, proper identification includes, but is not limited to, a valid passport for a child. The following are examples of additional documents that you may be asked to provide:

• documentary evidence or letter that proves the child has the permission of an absent lawful parent(s) or guardian to travel
• a copy of any separation, divorce or custody decree that proves that you have custody of the child
• a court order granting you guardianship of the child
• a copy of the child’s birth certificate, particularly if only one parent’s name appears on the birth certificate, and the child is travelling with the other parent.