Yes, if you have a child born outside Australia and one of the parents was an Australian citizen at the time of the child’s birth, then the child is automatically recognized as an Australian citizen.
However, it is a requirement that after birth, the parents need to register their child’s Australian citizenship (ie. Citizenship by descent) at the nearest Australian Embassy or High Commission.
*Legal Disclaimer: Please note that this article is for general information only. Always seek professional legal advice in regards to your situation as every situation is different. Immigration legislation changes regularly and thus always, get up-to-date advice on your situation. Lodging a visa application is a judicial process and is based on the quality of legal advice and legal representation you receive. It is illegal for any Migration Agent or Lawyer to guarantee 100% of a positive result as it is a judicial process.