The birth defect imposes limitations on her ability to use her lower limbs, although she is a "least severe" case on the scale of 1 to 10 - an S9 in para swimming for the disabled.
But McKelvie didn't let that inhibit her involvement with the netball team.
She put her hand up to be team manager in year 6 and the teacher rewarded not only her enthusiasm but her ability to think outside the square.
"I could not play but I was still part of the team," says the eloquent 15-year-old who warms to the interview as it wears on, breaking out of her cocoon of shyness.
For the record, she had been frequenting the Onekawa swimming pool from the age of 3 for group lessons, with sister Leigh, now 12, joining her not long after.
She took part in her first competitive swim meet in the final year of primary school.
It was imperative at year 6 she join a club, Greendale Swim Club, where coach Noel Hardgrave-Booth had a resume for mentoring children with disabilities into Paralympics material.
However, McKelvie had to put sports on the backburner when she attended Tamatea Intermediate.
Explains mother Michelle: "Kate had two major surgeries."
In April 2013, at the Independent Games (now called Junior Disability Games) in Timaru, a couple of talent scouts approached her to attend a talent ID camp in the southern town.
She did, three months later.
"I couldn't dive off the box the first time," she says of her first competitive swim during practice.
But it all became mind over matter for the youngster who joined the NGHS swim team with teacher/co-ordinator Louise Bassett-Foss at the helm.
"She was the first one ever, I believe [with a disability]," Michelle says.
In September 2013, McKelvie won two bronze medals in 50m freestyle and backstroke at the NZ Secondary Schools' Championship in her maiden competition meeting.
She went on to register a New Zealand Open qualifying time in the 400m freestyle in Gisborne, clocking 6m25s this month.
McKelvie also got NZ Open times in 100m backstroke and freestyle.
"They [sprints] are her main events but she's building towards middle-distance," Michelle says.
The intervention of the Halberg Disability Sport Foundation from 2013, with Laura Hill as the regional adviser, was pivotal.
"It's fantastic funding. We're into the second round now and Kate had one-on-one Pilates classes with Rachel Ward [Align Pilates and Physio]," the mother says of the instructor who carried out mat work to "strengthen Kate's core [abdominal area]".
Erin Fitzgerald is the new Halberg sport adviser who covers the Wellington/Bay area and met the swimmer and her mother for the first time on Monday in Napier.
McKelvie found impetus in the pool.
"Before, my legs were swaying everywhere in the water but now they are steady and I notice the difference in speed."
That core strength has enabled McKelvie to add the 100m butterfly to her repertoire.
Says Michelle: "It's another stroke she can take to racing."
Hardgrave-Booth pushes her out of her comfort zone although some days she feels it and finds an early night helps her rejuvenate.
Next month McKelvie jets to Sydney for the six-day New South Wales Open Championship.
"It's my first time overseas competing so I'll be internationally classified and can compete with whoever in the world," she says of the step up from national classification as part of a six-person Paralympic development team.
McKelvie also has forged closer ties with teammate Olivia Upton, of Auckland, who has cerebral palsy.
Says Michelle: "She's [Kate] learned to adapt to some things you can do and can't do."
The Bay co-ordinator for junior games (8-21 years), the senior McKelvie enjoys her role involving those with physical disabilities and visual impairment.
"It's not about competing but to have a go and have fun."
Kate McKelvie wants to compete at the 2020 Tokyo Paralympics and is in the phase of making plans under the tutelage of Gary Francis, the national development coach.
"It'll be a balance between school and sport for Kate," says Michelle of a daughter who is eeyeing a degree in the medical field.
The teenager, who has a Variety NZ gold scholarship for travel/gear/coaching, has received help from many people and swim apparel company blueseventy subsidising her gear.
