Watch: Lucy Lawless: Get behind Help Our Kids
She and her parents thought little of her lack of energy until April 2011, when the problem got worse.
Casey visited a GP, who detected a heart murmur and referred her to a heart clinic.
"When I saw the cardiologist, I was told if I didn't have surgery I wouldn't make it to Christmas that year."
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She had open-heart surgery at Starship in July 2011.
A normal heart has two coronary arteries supplying it with freshly oxygenated blood. Casey was born with one of these arteries wrongly connected so that it supplied deoxygenated blood. Only one or two babies are born with this defect in New Zealand every year.
In open-heart surgery, Casey's left coronary artery was severed from its faulty connection point and attached to its proper source, the aorta. The hole where it was moved from - on the pulmonary artery, which goes to the lungs - was patched with a Gore-Tex material.
Starship cardiologist Nigel Wilson says Casey's heart muscle "was becoming damaged but the operation was done in time and she is now doing very well".
Casey says she has much more energy since the operation and realises how greatly the undiagnosed heart condition affected her childhood.
"Even in cross-country [at school] I always started off first but would rapidly be the one coming in last. Or not being able to go to sleepovers because I felt exhausted and was in bed before everyone else.
"In photos now you can see a huge difference in the colour of my face and my skin - it's not pale."
Casey is speaking about her treatment to encourage Herald readers to donate to the Help Our Kids campaign to raise money for high-tech equipment in the new operating theatre being built at Starship and the four theatres being refurbished.
By last night, the campaign, run with the Starship Foundation, had raised over $137,000 from more than 1400 donations.
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