He won gold medal with the New Zealand team in the men's rugby sevens tournament during the 2018 Commonwealth Games. He was also a key member of the New Zealand side which won the 2018 Rugby World Cup Sevens tournament by defeating England 33-12 in the final.[4] He was named in the New Zealand squad to compete at the 2020 Summer Olympics in the men's rugby sevens tournament.[5] He was also part of the New Zealand side which claimed silver medal after losing to Fiji 24-12 at the 2020 Summer Olympics.[6][7] It was also New Zealand's first ever Olympic medal in the men's rugby sevens.
Knewstubb was named as a late signing for Tasman during the 2021 Bunnings NPC after a season-ending injury to Mark Tele'a. He made his debut for Tasman against Bay of Plenty at Trafalgar Park in a non competition match, starting in the number 15 jersey and scoring a try in a 26–9 win for the Mako. The side went on to make the premiership final before losing 23–20 to Waikato.