Depends when you were born. You might need your parents birth certificates or something. My husband was born in Australia but because it was after 1986, we needed to provide his mothers birth certificate AND marriage certificate to prove her name changed from maiden to married name on his birth certificate... This requested by immigration to obtain our daughters citizenship by descent. The quote below is from their request for more info.
"People born in Australia on or after 20 August 1986 are automatically Australian citizens only if at least one parent is an Australian citizen or Australian permanent resident at the time of the person’s birth in Australia."