Vanessa Heleta runs the Talitha Project which is trying to get a change in the marriage laws with its "Let Girls be Girls!" initiative.
Tonga currently allows people between the ages of 15 and 17 to get married if they have their parents' consent.
Ms Heleta said in 2015 there were 56 child marriages, as they are known in Tonga, and in 2016 there were 52.
She said 15 child marriages have been registered so far this year.
"It's really saddened my heart. I don't think it's right. This day and age we should not encourage child marriages," she said.