The Bethlehem College student said late night training sessions aided her success but admitted she was still "shocked" with her placings.
"I had worked really hard leading up to this but I didn't see it coming," she said. "I really surprised myself, especially in the 10km - I was racing against the likes of Charlotte Webby who just missed out on the Olympics, that's pretty cool.
"I was so happy with how I performed and that helped me going into the second day of competition."
Swim Rotorua teammate Milla also impressed competing in the 5km field - winning a National Secondary Schools' gold in the 13-years age group and a national silver in the 13-15 age group.
Milla, a student at John Paul College, said she was delighted with her performance.
"I was very happy with myself," she said. "I wanted to try my hardest and I was going for a title but I didn't expect to actually get it."
Swim Rotorua has a history of producing talented long distance swimmers including Olympian and five-time New Zealand Open Water champion Kane Radford.
Swim Rotorua assistant coach and Radford's mum Bronwen Radford said both Emily and Milla had the potential to "go far" in the sport.
"These two are certainly on the pathway to a great future," she said. "The key for them will be putting in the hard work over 800m and 1500m in the pool. That then means they can make a good transition from pool swimming to open water.
"Wouldn't it be really cool if we could produce a woman national open water champion? It's definitely a lot of hard work but there's a great opportunity there."
Both Emily and Milla were part of the AquaKnights Team, which consisted of the best swimmers from the Bay of Plenty, Waikato, Hawke's Bay/Poverty Bay and Taranaki, at the Victoria State Age Groups in Melbourne at the end of last year.
Milla was a standout for her side after she stormed her 800m freestyle heat to take silver in a new Bay of Plenty record time of 9m 30.40s.
For the rest of this season the Swim Rotorua pair were both targeting success at this weekend's 2km Blue Lake title, the first of four Swim the Lake Rotorua challenges.
And then making a splash at the New Zealand Age Group Championships in Wellington in March.
With the long-term aim for both local swimmers to represent their country - in the open water field for Emily whilst in the pool for Milla.
"I want to keep challenging myself and swimming is what I want to keep on doing," said Emily. "Kane [Radford] is a real inspiration of mine and I'd also love to represent my country. I may have to sacrifice a lot a long the way but that's what it takes."
Milla added: "I would love to go to the Olympics in the pool. At the moment my strengths are the 1500m and 800m freestyle and the 200m butterfly - I want to see how far I can go with these."
