Out of curiosity I tried this procedure and answering the quiz
Was the baby born in AU ? No
Was at least one parents an AU citizen at the time of the baby's birth ? Yes
View attachment 3145
And it is led to this site
Find out about Australian visas, immigration and citizenship.
immi.homeaffairs.gov.au
You must have been born outside Australia
ensure that the person you are claiming descent from was your parent at the time of your birth and was also an Australian citizen at the time of your birth
be of good character if you are 18 years old or over when you apply
You could even apply online
Find out about Australian visas, immigration and citizenship.
immi.homeaffairs.gov.au
The way I understand it. It does not say you or the Baby will need to be in Australia at the time of application.
Even a son already over 18 years old could still even apply by himself from outside AU as long as they have proof/ evidence on what is needed.
What about asking directly to home affairs ??