Sandi Gardiner was born in Adelaide, Australia, to Carina Calo, a civil engineer, and Leonard Albert Gardiner a microbiology professor. She is of Philippine, Chinese, Native American (mother) and Welsh, English (father) descent. Sandi grew up traveling the world as an only child with each of her parents. Her mother schooled her in Asian culture and language and her father in European culture and language. Sandi began modeling at the age of 8, when a modeling agent stopped her mother in the street in Australia and she went on a casting that same day. She booked the job and continued to model all throughout primary school and high school.