Last year, South Korea passed the law, an amendment to the Telecommunications Business Act, which bans major app store operators such as Google and Apple from forcing software developers to use their payment systems.
The rules have been in effect since March.
The Korea Communications Commissions said in a statement it had conducted an inspection since May 17 to determine whether Google, Apple and One Store had violated the revised Telecommunications Business Act. It determined that all three app operators might have violated the rules.
Google, Apple and One Store were not immediately available for comment when contacted by Reuters.
The KCC added that it plans to take strict measures such as correction orders or imposing fines if the probe finds barred activities.
Barred acts include app market operators unfairly delaying the review of mobile content, or refusing, delaying, restricting, deleting, or blocking the registration, renewal, or inspection of mobile content that uses third-party payment methods.
Potential fines for infractions will go as high as 2% of the average annual revenue from related business practices, the rules say.
(Reporting by Heekyong Yang; Editing by David Holmes and Louise Heavens)