The Royal Forest Department said it plans to investigate a company that has been criticised for operating a cafe restaurant close to a waterfall inside a nationally protected forest in Chiang Mai.

Attapon Charoenchansa, deputy chief of the department, said the the restaurant operator has been charged for operating a zipline in Mae Rim National Reserved Forest, in Mae Rim district of this northern province.

Mr Attapon made the comment after an image of the scenic property near the waterfall went viral on social media in the last week. Visitors can sip coffee there while taking in panoramic views of the waterfall.

Mr Attapon said the company has been operating the business under the name Pongyang Zipline & Jungle Coaster.

Last October the same operator was found guilty of encroaching on 3 rai in the forest and was ordered to remove the zipline and restaurant, which were deemed illegal.

It began demolishing the zipline structure but not the other property, he said.

This is one of 12 illegal ziplines the department has detected.

Seven now face legal action, three have been dismantled and one is undergoing verification of its land documents, authorities said.

The department will visit the site of Pongyang Zipline & Jungle Coaster soon to continue its investigation, according to Cheevapab Cheewatham, who holds a chief-level position there.

He said the agency has been aware of the case since October but has not pressed charges as the owner was first given the chance to present his land documents to the authorities.

"The owner is apparently now starting to demolish part of the property as he may have realised he in fact has encroached on the forest," Mr Cheevapab said.

"We intend to take legal action. He should never have been allowed to run a restaurant in a location where that is prohibited."

The operator could not be reached for comment during press time.

Mr Cheevapab said the department must next ask the Department of Lands to help verify the land title document of the cafe restaurant.

The Royal Forest Department said the owner must have violated the law as there is no way for individuals to obtain land title deeds near a public waterfall.