Mayor Bill de Blasio promised to run the most transparent administration in Big Apple history, but City Hall appears to have violated the state’s open records law by withholding key data about the East River ferry, experts said Wednesday.

In April, The Post demanded the statistics compiled by the Economic Development Corporation about the $637 million NYC Ferry system’s ridership under the state’s Freedom of Information Law — only to be repeatedly rebuffed and told the search for the records was still ongoing.

However, EDC revealed the existence of the data by leaking a presentation summarizing its findings to another newspaper Monday night.

“It violates the Freedom of Information Law,” said John Kaehny, an open-records expert who runs the good-government group Reinvent Albany. “It’s really unacceptable — and particularly lame.”

The Post filed the FOIL request on April 15 asking for “the results of the demographic survey of boat riders, by line and by stop, if available,” as well as for contracts and other related documents.

The agency confirmed it received the records request on April 22 and later released the contract and some supporting paperwork, but never provided the crucial ridership data.

Instead, it claimed in letters dated May 20, July 5, Aug. 2 and Sept. 6 that agency officials were “continuing to search for additional documents.”

However, EDC’s 13-page presentation summary reveals that as it stonewalled The Post and others, it had already conducted at least three surveys of its ridership.

“This is the 4th onboard survey conducted in NYC Ferry’s 2 years of service,” it disclosed on the second page of the presentation.

The document shows that the previous surveys covered summer 2017, winter 2018 and summer 2018. The most recent survey covered in the memo examined ridership from this summer.

“This is really not acceptable conduct by any government agency,” said Susan Lerner, the head of watchdog group Common Cause New York. “The data is subject to FOIL, there is no excuse for playing games.”

“Certainly the spirit of the law has been violated,” she added. “This sort of game-playing should not be countenanced.”

The agency still has not provided The Post with an official response to the legally binding records request, despite delivering the documents to another paper Monday night.

“EDC’s outright resistance to these transparency requests is outright shameful,” Kaehny added.

The Post isn’t alone.

The agency also blew off City Councilman Donovan Richards (D-Queens) when he asked for more information about adding a second ferry stop in his district, which includes Far Rockaway. He compared getting data out of the agency to “beating your head against the wall.”

He also blasted EDC’s refusal to provide documents to The Post and other reporters.

“I want to thank the New York Post for digging on this. It points to a reality that unfortunately, as we try to address this Tale of Two Cities, this shows we are still not where we need to be,” Richards said. “How much subsidy and how much infrastructure are you putting into the communities that really need it?”

EDC defended its response to the paper’s FOIL request, insisting it did not run afoul of the law.

“NYCEDC takes its FOIL obligations seriously and rejects any suggestion that it did not abide by the law,” said spokeswoman Danielle Schlanger.

Additional reporting by David Meyer