Adoption is rare in Australia. That needs to be stated up front. It does happen occasionally, but local adoption mostly happens through the Permanent Care system, meaning there are children in the foster system who will need a permanent family. It is rare that a parent place their child up for adoption at birth.
Inter-country adoption is available from a small number of countries, with various rules for Australians wishing to adopt.
A general rule across the board in regard to adoption is that any biological children must be 2 years or older before any process of application will be considered, and your youngest existing child must be at least 2 years older than any child you adopt.
The adoption process itself requires much patience, as it will take between 3-10 years from application before a child from another country is placed with an Australian family.
The greatest need for adoption is for children with complex medical conditions.
Anyone seeking to adopt should enter the process desiring to provide a family for a child, rather than seeking a child for their family.
Firstly, consider whether local permanent care could be an option for your family. You can find out more here.
Or you can begin an application of interest for Inter-Country Adoption by visiting the Australian government adoption website here: