Perhaps the physical and mental toll of the past year, as she fought her way back from injury, is beginning to show. Maybe her mind is beginning to shift to what lies ahead when she steps away from the game.
But there are still glimpses of magic. On the training court against a team of promising youngsters from the Victorian Institute of Sport, Kopua is giving a demonstration of her knack for spectacular aerial intercepts - something she has turned into an art form.
A long, cross-court pass from the Victorian centre looks destined to find its way into her shooter's hands, until Kopua floats out of nowhere to snaffle the ball. She makes it look effortless, gracefully swooping on the ball, plucking it out of the air and offloading to Laura Langman in one fluid motion.
"I'm still here," she says, loitering at the transverse line, ready to back up if there are no passing options further up court.
Before the game gets underway Kopua has a friendly chat with the young shooter, helping put her at ease. It must be daunting for a teenager to have to play against the Silver Ferns captain and one of the greats of the game. Kopua knows this.
Her team-mates will tell you one of Kopua's greatest strength as a captain is her intuition. The instinct she shows on the court is also evident off it. She'll recognise when someone may be having a rough day or is struggling, and make it her business to help.
Katrina Grant, the captain's defensive sidekick and close friend, said even when they played together at age-group level Kopua was always the "camp mum". Grant recalls rooming with Kopua when the now Central Pulse captain joined the New Zealand under-21 side in 2004.
I was still in my sixth form year at school and so it was pretty daunting to come into this team with players that were a bit older than me. Casey always made sure you had everything you needed and you weren't going to get in trouble for forgetting something or being late or anything like that.
If you watch closely in the pre-test ceremonies, Kopua's protective instincts reveal themselves.
When the players line up for the national anthems, they stand with their arms linked. Kopua, however, stretches out her arms, wrapping them around as many of her team-mates as her 1.92m wingspan allows, as if to say "I've got your backs". Netball New Zealand communications manager Kerry Manders puts it this way: "It's like she holds the team together in both the literal and figurative sense."
Grant says Kopua's nurturing nature extends beyond netball. Grant, a bridesmaid at Kopua and husband Terry's 2012 wedding, jokes the bride spent more time looking after her than the other way around.
"It was her wedding, but she was more focused on us and making sure we were happy and comfortable. The night before, she didn't want anyone to be by themselves so she brought all the mattresses out and stuck them in the lounge, so we were all together."
It might come as a surprise to Kopua's opponents over the years to learn the defensive warrior is so considerate, because on court she takes no prisoners.
She may have been kind to the young shooter before the training game got underway but, as soon as the whistle sounds, Kopua does everything she can to stop the ball from going through the net.
The opposition score just five goals in the period, but at the quarter-time break Kopua delays joining the team huddle while she has an intense debrief with defensive partner Leana de Bruin, who also looks set to step down at the end of the series.
It's as if each of those five goals personally pains them. The pair animatedly dissect where they might have gone wrong, and how they could do things differently next time.
But Kopua is running out of next times.
She hasn't said so officially, but the rumour mill has it the final Constellation Cup test in Perth will likely be her last.
The way the brilliant Diamonds are playing right now, it may not be the swansong Kopua hoped for or deserves. By now, however, she is familiar with the cruel nature of the sport.
One of the game's most gifted players, Kopua is also genetically cursed, with her career regularly stalled by a degenerative condition in both knees and dodgy ankles.
In spite of this, she is New Zealand's most-capped skipper of all time, and widely regarded as the greatest defender the country has produced. She may not walk away from netball with world titles and championship glory. But she will bow out with 100 tests and an unrivalled legacy.
