He was diagnosed with pulmonary stenosis - a condition in which two arteries of the heart aren't the same size.
"If they were the same size things would have been easy and they would have been able to switch the arteries over, which was the plan.
"All of a sudden the surgical plans were totally thrown out the window, so we really didn't know what was going to happen.
"He had open heart surgery when he was a week old and then he had his second one in March.
"They managed to do quite ground-breaking surgery ... it was one of the most difficult surgeries that they've had to do," Ms Harrison said.
The doctors had told her Lyndon would not have to have another surgery for about a year but would need regular checks for the rest of his life.
Ms Harrison said Heart Kids, a charity, had provided amazing support while Lyndon was in hospital.
"You just don't really know what you're going to need when you're in that situation but to have them there every day was so good. They were incredible.
"Heart Kids has been amazing to us. We have only been [in Rotorua] a few months and we already know Heart Kids here and some other families.
"I don't know what we would've done without them.
Lyndon was growing really well, "like a normal baby", she said.
"We are just really lucky. There's some babies that didn't make it. We are just really grateful.
"He's not going to be an All Black but he might play golf."
Ms Harrison encouraged locals to support the annual Heart Stopper Challenge, a fundraiser for the local branch of Heart Kids on this weekend.
