McCartney laughs easily, and often. Then again she's got plenty to be happy about. Five years after taking up a sport rich in the complexity of its various components, she's preparing to compete on sports' grandest stage against its best exponents. That's some leap.
To get to Rio, McCartney has pulled off what racegoers would call a big rails run. She won the world junior title last December, then produced a national record, and personal best, 4.80m at the New Zealand champs in Dunedin to qualify for Rio, then took fifth place at the world indoor championships in Portland, Oregon, in May.
McCartney admits to surprise at her progress.
"I have to say my coach [Jeremy McColl] has had big dreams, and always told me what he thought I could do, and a lot of the time I'd say, yeah okay.
"But a lot of the time he's bang on, on what he thought I was going to achieve. So I've learnt to trust him a lot more, and learnt to trust my ability."
McColl, a former New Zealand gymnastics representative and a pole vaulter at Oceania championship level, remembers first sighting McCartney.
"I was coaching a boy at her school [Takapuna Grammar] and Eliza had been doing high jumping. He mentioned she was keen to try it.
"She was pretty much a natural from the start," McColl said.
The young McCartney loved her netball and tried her hand at a range of sports. She saw the pole vaulters and "being a curious kid, and pretty adventurous, I wanted to give it a go. It looked like fun. I went with a friend, and didn't stop".
So what is it about pole vaulting that spins her wheels?
"One thing was I quickly started doing well, thanks to Jeremy.
"I was surprised by that and kept going. It is technically a hard sport. There are a lot of components to it. It can take years to get that technique right. It is something that when you do it right, it feels amazing."
This is not a student who sat in class and stared absent-mindedly out the window at the playing fields, her mind drifting away.
"I really enjoyed my school work. I was really into science and did strive hard to get results in my exams. I've always loved the academic side."
Bachelors of science and physiology most likely figure in McCartney's future. She loves her sport, but is keenly aware there is more to achieve in life.
That McCartney - who happily confesses to having a love of chocolate, which needs careful monitoring - relies heavily on McColl is clear.
"He knows everything there is to know about the pole vault and I certainly don't. He knows so much about technique and training.
"It's one of those sports where there's so many aspects it's good to have someone who's an expert in the field."
So for those who've never had the inclination, talent or guts to propel themselves close to 6m straight up and over a bar, what's it like? McCartney tries to keep things simple.
"At the beginning I really want to focus on what I'm doing, take a moment to block everything out and get my head into what I'm doing and visualise exactly what I want to do. I focus on only two or three components because there's too much to think about."
She reckons she's good at shutting out distractions.
"It's funny but when you're really focusing on what you need to do in a jump, you block everything around you. You've got this narrow vision to the box [where the pole is planted] so it comes naturally.
"I try to visualise and feel it before I take off so I can nail the jump."
And when you land? All depends on what happened half a second earlier.
"Rats or elation," McCartney laughed. "As soon as you're falling you know where the bar is. It's raw emotion and you're in the moment."
That this is a complex sport was apparent during a training session recently. Several times, McCartney ran up to plant her pole before taking off, and pulled out. Her pacing was slightly out, therefore there was no point carrying through. Small elements can count for plenty.
McCartney is exceedingly grounded. Family and friends are good for that. No getting ahead of herself. Her heroes have been shot put legend Valerie Adams and more recently kayaker Lisa Carrington.
"Valerie handed me my medals at a Weet-Bix [youngsters] triathlon. I remember that very well. I wasn't even doing athletics back then. It's incredible watching her, and Lisa, such powerful, strong women and so respected."
And now they're teammates in the wider New Zealand team? "It's weird, still a bit funny to think about."
So what would constitute a successful Olympics for this bright, bubbly and gifted athlete? There's no bold talk of medals, although she laughed that she wouldn't say no.
"Having the best technical competition I've ever had. It's about bettering yourself.
"It's so important I work really hard to get the process right. I can't control what the other girls jump on the day. But to be technically the best I can, that will make me happy."
Coach McColl reckons McCartney has made significant strides this year. Her run up and take off is now more natural and she is getting more energy into her work. So what about that smile, the laughter?
"It's very rare you don't see that. It's just the way she is. There's always a time when the game face does on, but she really enjoys the process of it all. It doesn't overwhelm her.
"She enjoys putting the whole puzzle together. She's highly intelligent so she can see how one thing leads to another and how it all links."
McCartney has learned to handle an increased spotlight in the past few months, McColl said, "but she's still Eliza. She hasn't changed".
