Jonas also performed well in the open men's division, making it through to the semifinals where he finishing fifth, just outside the final four.
"I almost made the final which was probably my best result so far, so I was pretty amped about that," Jonas said.
He says winning the under-18 title "is a pretty big deal for me and I am pretty happy with how it turned out".
"I was pretty stoked. I didn't really think about the outcome. I just had to surf and it was pretty big and messy. I was sitting further out from everyone behind the 'beehive', which is right out the back and made sure that I got the waves and then I surfed as hard as I could.
"I was just pumped as."
Elin knows she could so easily have won the under-18 girls' division.
"The final is over 25 minutes and it is pretty random really in Piha. I got a pretty good wave and got an 8. something so if you get that you pretty likely to win," she said.
"They were big and tricky conditions so it was hard to get waves. I just needed a better back-up wave and got a 4. something and I needed a 6.
"I was close but you always need that little bit of extra time. I am still happy with second but would definitely have liked to have won."
Jonas has aspirations to be a pro surfer at some stage and rates fellow Mount Maunganui surfer Kehu Butler as an inspiration.
"I am definitely amped up to give the pro juniors this year a shot because I am still under 18. They are in Australia and run through the east coast through the Sydney beaches in the first half of the year. That is pretty much the goal.
"Kehu Butler is doing them, he is right in there. He is pretty legendary. He is pretty good at competitive surfing, he has huge family support and pretty good sponsorship deals too.
"He rips. He is such a good surfer and I definitely look up to him."
Elin has similar goals for the future but says funding is quite difficult as it is very expensive.
"I definitely want to go over there and try those because that is the next step for both of us."