'7th Heaven' Dad Stephen Collins - Confesses on Tape to Child Molestation ... NYPD Investigating

Stephen Collins -- who played the pastor/dad on "7th Heaven" -- confessed to his estranged wife he was a child molester, and it's all on tape ... a tape obtained by TMZ. And the New York Police Dept. is now conducting an active criminal investigation involving sexual contact with multiple children.

Collins -- who is in the middle of a nasty, prolonged divorce with actress Faye Grant -- revealed to her in 2012 he had molested and/or exposed himself to several underage girls years before. Collins and Grant went to a therapist where she peppered him with questions about the incidents ... and he not only answered, he was specific.

Here's what Collins didn't know at the time. Grant taped the therapy session. We're told her lawyer advised her it was legal to secretly record the conversation because in California you're allowed to secretly record conversations to gather evidence the other person committed a violent felony ... and molesting a child under the age of 14 qualifies.

We have protected the names of the victims Collins mentions on tape. As you listen ... you will hear Collins refer to a "disclosure" and a "list." This refers to a confession Collins had already made to his wife ... which triggered the therapy session.

You hear Collins flatly confess to molesting an 11-year-old New York girl -- a relative of his first wife -- saying, "There was one moment of touching where her hand, I put her hand on my penis." He also acknowledges exposing himself to the girl "a couple of times" ... he says when she was 11, 12 and 13.

Grant asks, "When you exposed yourself ... did you have an erection?" He responds, "No, I mean, no. Partial, maybe I think."

Grant then inquires about other girls. Collins mentions an L.A. girl who lived in their neighborhood, but says he tried righting the wrong by apologizing to her years later.

And then they talk about yet another girl. It's incredible ... Collins has trouble keeping the girls straight, but then he gets clear and confirms the incident. We're told this girl was also from New York and was between 12 and 13 at the time.

TMZ has learned ... the NYPD is in possession of the tape, and last Wednesday 2 NYPD detectives from the Special Victims Unit flew to L.A. to interview Grant. We're told they asked her about the tape and about other potential witnesses.

An NYPD official tells TMZ ... there is a current, active investigation. The official says there are "at least 3 victims" and the nature of the investigation is "inappropriate sexual contact with minors."

Law enforcement sources tell us ... one alleged victim in the investigation is the relative of Collins' first wife, who we're told first reported the molestation in 2012. One law enforcement source tells us ... although decades have passed since the molestation, they believe the case can still be prosecuted because the statute of limitations runs from the date of reporting.

Collins -- who is on the National Board of the Screen Actors Guild -- frequently interacts with young actresses. In addition to "7th Heaven," he was also on the ABC Family show "The Fosters," about a bunch of young kids being raised in a foster family. And he had a role in the recent show, "Revolution" ... which also had underage actors.

We contacted Collins' reps ... they had no comment.