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Kopua made a shock return to the Silver Ferns last year when Noeline Taurua was appointed as new head coach of the national side.
But Kopua, who looks set for July's Netball World Cup, said this year would be her last.
"This next six months is the biggest six months for me because after that, that's it. The only difference is this time it's for good. My mind is willing, but my body isn't," Kopua said.
"I have had the best career with Magic. I have grown as a person and a player. Thanks so much for all the opportunities, all the friendships and all the support over the years. Magic will always be a part of me."
Celebrating 20 years as a franchise, 2019 was always going to be special for the Magic, but it will now be even more significant.
Kopua has etched a rich legacy with the sport and is the embodiment of the Magic culture and ethos.
Kopua has only ever played for Magic, joining the team as a teenager (alongside Laura Langman) in 2003. She has been the rock of the team's defence since, becoming one of the sport's best ever defenders, a Silver Ferns captain and centurion.
With Magic, Kopua won two National Bank Cup titles, an ANZ Championship title and two ANZ Championship New Zealand conference titles.
Magic head coach Amigene Metcalfe has known Kopua as a team-mate and as a player under her charge.
"Casey is certainly synonymous with Magic Netball and it's with great pride I've been able to watch her growth as a team-mate, coach and friend.
"From early on it was obvious to see her potential but what she has given to netball in the region, nationally and internationally has been huge. She is a true competitor, a loyal team member and I'm sure she'll be determined to make this year her best one yet."
Netball Waikato Bay of Plenty Chief Executive Rohan West also paid tribute to Kopua.
"Casey is Magic and Magic is Casey. She's a living legend. It's as simple as that. The history and legacy of this team wouldn't be what it is without her. It's been an absolute pleasure working with her," he said.
"The phrase Mana Wahine could have been coined for Casey. I strongly urge all Magic supporters, and general sports fans, to come to our home games this season to see Casey in action one last time. I'm not sure we'll see the likes of her again for many years, if ever."