"She looked pretty cold, but I told her everything was going to be alright. I put the life jacket on her and we drifted back."
Spitzer swam about 200m back downstream to the riverbank where firefighters were waiting. He said people often underestimated the strength of the river's current.
It was the first time he'd had to put his swimming talents to the test, he had never had to rescue a victim from the river before.
The girl was "quite hypothermic and buggered" Lapwood said.
"The police were there and our brigade got called out and we considered getting the rescue boat from Huntly, but just due to time and the person's condition they decided, because we've got a top swimmer in our brigade, not to."
A St John spokeswoman said the young girl was taken to Waikato Hospital in a serious condition.
Lapwood was proud of the volunteer's effort.
"It's a good effort. Quite often these jobs you're put under the pump to make a decision one way or another and if you've got the skills or the training and the confidence to swim, well it's a call you make at the time."
"In my opinion he's certainly got the capability to perform that task. He's a bit of a machine this boy. He got second in the Sky Tower Challenge so he's certainly a well-rounded fit guy as far as athletics go.
"It's quite a swift river and once you get a few logs around the piers it does have an element of danger to it. You can't just let anyone go in there, you need someone who has got really proficient swimming skills who's had experience swimming in the river."
He urged people to take care if planning on swimming in the Waikato River this summer.
"It is a very dangerous playground, so take all extreme precaution as much as you can and if you are going to swim in there, swim with your friends so that people can keep an eye on you."
Northern Fire Communications spokesman Paul Radden earlier said they received a call about 12.15pm from a concerned member of the public who said the child appeared to be struggling.
The child was hanging on to supports of the State Highway 1 Ngaruawahia bridge and was unable to get back to shore, Radden said.