There are still debates in Cambodia about allowing out of country voting (see Source). However, as of April 2018 the NEC website does not contain any information about the adoption of specific regulations or amendments in the law in this regard.
"Som Sorida, deputy secretary-general at the NEC, said 536,230 eligible voters had registered out of a target of 1.6 million. “Many of the 1.6 million people are migrant workers abroad,” Som Sorida told Reuters. “We don’t expect that they would come to register to vote.”"