Born in Pukekohe, Forbes first caught the eye when helping Bay of Plenty to the Jock Hobbs Memorial National U19 tournament in New Zealand, before going on to make his senior debut for the province in the Mitre10 Cup.
Scottish-qualified courtesy of his grandfather, Forbes was part of the New Zealand squad for the 2019 World Rugby U20 Championship alongside future All Black Tupou Vaa'i and Super Rugby Aotearoa winners Cullen Grace and Leicester Fainga'anuku.
Also comfortable playing at fly-half, he made three appearances in the tournament including starting two of New Zealand's three group stage matches.