Seila Samleang, the head of Action pour les Enfants (APLE), first tracked 72-year-old David Lavigne four years ago.
APLE works to help victims of sexual abuse and labour exploitation in South-East Asia.
Samleang says Canadian authorities failed to reply after he alerted the RCMP that Lavigne was allegedly abusing two sisters.
"No question, no call, no conversation, no discussion, no interest," said Samleang.
One of the girls in question, Map, was 9-years-old when Lavigne first met her as she was selling trinkets on the streets. She said Lavigne showered her and her sister with gifts and took them on vacation.
"I don't know when he started hurting me," she said. "But I didn't cry."
Samleang put pressure on Cambodian authorities to arrest Lavigne who ended up serving time in Cambodia until the RCMP Canadian Police Centre for Missing and Exploited Children deported him back to Canada in October 2012 where RCMP executed two outstanding arrest-warrants for alleged child sexual offences in Ontario.
These allegations, which the RCMP said were uncovered during an investigation in March 2012, would have taken place before Lavigne left for Cambodia.
Sergio Pasin, the RCMP's director of international operations, said the unit may have failed to answer Samleang's concerns.
"There may have been a lack of communication in the file and there was no follow up," he said.
The RCMP runs a special squad that is supposed to enforce Canada's law against sex tourism in other countries. The law was adopted more than 15 years ago to help fight sexual abuse of minors.
Despite the law, Canada has one of the worst records when it comes to cracking down on sex tourism, lagging far behind the United States and Australia, which have similar laws.