The Department of Groundwater Resources is inspecting business premises in Bangkok after receiving complaints of illegal groundwater consumption, including by massage parlours.
Department deputy chief Aranya Fung-swadi said many businesses had been illegally drawing groundwater to cut costs.
He had sent staff to probe complaints by collecting water samples and sending them to a laboratory to be tested.
Many complaints were substantiated. "We have found many water samples were of underground water origin, not tap water as claimed by the business operators.
"However, the massage parlour shops are not our prime target to examine," she said, adding wrongdoers had already been fined.
The penalty fee per individual case varies, but does not exceed 10,000 baht.
Thailand has been plagued with groundwater consumption problems which lead to land subsidence across the country.
The department has formed a special task force, "The Underground Water Protection Centre", to examine illegal underground water consumption nationwide.
The hot spots of illegal consumption are areas populated with resorts in Khao Yai area in Nakhon Ratchasima, and factory areas in Khon Kaen province.
Bangkok was among areas plagued with land subsidence caused by massive underground water consumption several decades ago, until the authority imposed restrictions in 1977.
The situation has improved, according the the department, as there are less than than 1,000 users in Bangkok and its surrounds, consuming a total of 300,000 cubic metres daily.
That's a small fraction in comparison to the total 1.25 million cu m of water going to replenish the underground aquifer.
The authority has been imposing price mechanisms to deter consumption. For examples, the price of underground water consumption in Bangkok and seven provinces around is 13 baht per cu m (8.50 baht for water fee, plus 4.40 baht for conservation fee), while the rate for other provinces is 3.50 baht cu m.