Beyer was born in 1957 at Wellington Hospital to Noeline (née Tamati) and Jack Bertrand and assigned male at birth. They named her after her maternal grandfather, Lieutenant Colonel George Bertrand, who was second in command in the Māori Battalion. Her parents were living in Hataitai at the time of her birth. She is of European and Māori (Te Āti Awa, Ngāti Mutunga, Ngāti Raukawa, and Ngāti Porou) descent. Her mother had a second child, Karen, to her first husband in December 1958. Karen was adopted out and few in the family knew that her mother had been pregnant a second time. Beyer was sent to live with her grandparents on their farm in Taranaki during this second pregnancy. Her parents divorced by 1962.