Children born in Australia automatically acquire Australian citizenship if at least one parent is an Australian citizen or permanent resident at the time of the child's birth.
Children born outside Australia to parents who are not Australian citizens do not automatically acquire Australian citizenship, nor are they eligible for citizenship by descent.
See: Proof of Citizenship
Children born outside Australia to permanent resident parents will need to apply for and be granted a permanent visa to be able to enter and remain in Australia with their parents.
Most children born outside Australia to permanent resident parents will need to apply for a Child (subclass 101) visa.
All people who apply for a Child (subclass 101) visa, including applicants who are the children of permanent residents, must meet the eligibility requirements and pay the associated charges for the relevant visa.
Further information about applying for citizenship is available in Becoming an Australian Citizen