Children who is born from an Australian citizen in Japan needs to apply for their citizenship at Australian embassy in Japan.

Japanese family register (‘Koseki Tohon’ – 戸籍謄本 or ‘Koseki Shohon’ – 戸籍抄本) is required to prove the relationship.
The document is written in Japanese so it should be translated and must be certified by a person with the appropriate authority. They must be:
 an Australian citizen, or
 if applying from outside Australia, a citizen of your country of residence if no Australian citizen is known to you

AND
 have known you for at least one year
 not be related to you by birth, marriage or de facto relationship
 be easy to contact by telephone during normal working hours

AND
who is on 38 listed occupation by Australian government.
http://japan.embassy.gov.au/files/seol/17%2001%20-%20CBD%20checklist%20-%20ENG.pdf
Notary public, Judge of a court and there is a Legal practitioner on No 15

15. Legal practitioner (licensed or registered)

And it is including Japanese immigration lawyer like us.
We take care of the translation of Japanese certifications.