You know City Hall has crossed the line when even City Council Speaker Melissa Mark-Viverito is squawking — as she is over Mayor de Blasio’s stonewall over the Rivington St. nursing-home flip.

At a press conference calling for new rules for lifting deed restrictions like the one in that now-notorious case, the speaker called it “very troubling” that the mayor’s office impeded the Department of Investigation’s probe of the affair.

“It’s very clear what the authority of DOI is. [It] is to have full access to documents — so that is a concern,” said Mark-Viverito.

It sure is. What is the mayor hiding?

After all, what DOI did uncover already shows he’s a liar. When word of the flip hit the press, he claimed City Hall had been blindsided by the news — yet The Post uncovered proof that two deputy mayors had been all over the deal.

He also insisted he first learned of the flip from the papers — yet DOI produced a memo showing he’d been briefed on it.

What else is to be learned from the documents and electronic devices that de Blasio’s Law Department refused to let DOI ininspect?

And why did DOI take “no” for an answer?

Councilman Rory Lancman (D-Queens) has a point: DOI chief Mark Peters — who was the finance chairman of de Blasio’s 2013 campaign — should resign if he won’t pursue legal action to get that “full access.”

US Attorney Preet Bharara, state Attorney General Eric Schneiderman and Manhattan DA Cy Vance have no doubt taken notice and are drafting subpoenas to take possession of the possibly incriminating information.

If DOI won’t get the job done, it falls to prosecutors to get to the bottom of the Rivington swindle.

Michael Benjamin