The religion question is one of the few which are optional. On the accompanying website, it is explained:
Why is it asked?
Church and religious organisations depend on the Census for information about how many people of their religion there are in different parts of Australia. They and others use the information to assess the need for religiously based schools, hospitals, community services and homes for the elderly.
How to answer the question:
Answering this question is optional.
• If a person’s religion is one of those listed, mark the box next to it.
• If a person’s religion is not listed, write the name of the religion in the ‘Other – please specify’ box.
• If a person’s religion is an Eastern Catholic religion such as Maronite Catholic, Melkite Catholic or Ukrainian Catholic, write the name of the religion in the ‘Other – please specify’ box.
• People who have non–theistic religious beliefs or other life philosophies should write their response in the ‘Other–please specify’ box.
• If a person identifies with no religion at all, mark 'No religion’.
I wonder how various kinds of Aboriginal people answer this.