Betham was born in
Wellington, New Zealand, to parents of Samoan heritage. His family moved to Australia and he was brought up in Sydney where he started playing rugby league with the
Coogee Randwick Wombats club at the age of 10. He played high school rugby for
St. Joseph's College, Hunters Hill located on Sydney's
Lower North Shore. He became friends with St. Joseph's teammate
Kurtley Beale and for two years the pair backed up with Kensington in the South Sydney junior rugby league competition. Having attended a public primary school, Peter represented Sydney East PSSA at the State Championships in 2001 at Kiama. His teammate Rob Horne and Peter went on to represent NSW PSSA that year. Betham played for NSW Schools in the 2005 National U16 Championship. He subsequently played for the Australia A Schools team that went on to beat New Zealand. In 2006 Betham toured
Fiji and New Zealand with the
Australian Schoolboys team. In 2007 he played rugby while studying American history at
Sydney University, he played rugby for Wesley College and was selected in the Australian U19 Rugby World Championship squad. He also became a holder of an Australian Institute of Sport (AIS) scholarship, one of 26. ==Rugby union career==