Koopu started playing rugby league for the Taniwharau Rugby League club in Huntly, where he attended school at Te Wharekura o Rakaumanga.[3] In 1998 he also represented the Waikato Cougars, played for the Junior Kiwis and was named in the National Māori tournament team after representing Waikato Māori.[4] During the early part of 1999, Koopu played for the Glenora Bears and represented Auckland North.[5] While contracted to the Warriors, Koopu also played in the then feeder clubs for the NRL franchise, the Wynnum Manly Seagulls and the Newtown Jets. Koopu also represented the NZ Maori at the 2000 RLWC and played for the Kiwi's 3 times.
Koopu played for the New Zealand Warriors from the interchange bench in their 2002 NRL Grand Final loss to the Sydney Roosters. It was not until 2004 that he made his debut for the New Zealand national rugby league team.[7] Koopu seriously injured his shoulder in 2006 and never fully regained his consistency with the Warriors.[8] As a result, after ten years with the Warriors, Koopu was released by the club at the end of the 2008 season. That year Koopu again played for the New Zealand Māori in two games, including a match against an Indigenous Dreamtime side.