As a result of the offences, the KSA has issued a €350,000 fine for LeoVegas and a €400,000 fine for Royal Panda.
A KSA investigation found sites for both companies were accessible with a Dutch IP address, while it was also possible to use Dutch iDEAL payment to play.
Online gaming is currently prohibited in the Netherlands, although applications for licenses will be accepted in 2020 after the Dutch Senate voted to approve the Remote Gaming Bill earlier this year.
The online market is expected to launch in 2021.
Upon passing the bill, René Jansen, KSA Chairman, commented: "Online gambling in the Netherlands is illegal.
"Under the current legal regime for companies, it is not possible to get a license for offering online games of chance. Consumers are not assured of a safe game on an honest market with illegal providers."
Other operators have received similar fines recently, after engaging in unlicensed activity in the country, with 1xBet receiving a €400,000 fine in February.
Gambling Insider understands LeoVegas is considering appealing the fine.
When contacted by Gambling Insider for comment, a spokesperson said: "We are considering to submit an appeal while awaiting for the regulatory reforms to enter into force and the remote gambling licensing process to commence.
"There is an ongoing consultation on secondary regulations and the KSA intends to publish further details on the licence application process."
