More than 100 of New York's Fin est were investigated last year for allegedly associating with criminals - from boozing with mobsters to pulling off robberies with gang members to even carrying on affairs with lowlife drug dealers, The Post has learned.

The figure represents about half of the serious Internal Affairs corruption investigations, according to sources.

"It is one of the big problems the Police Department faces," said a source familiar with the NYPD's hard-charging efforts to root out wrongdoers.

In one astounding case, transit cop Yaniris Balbuena, 31, was charged this month with laundering $250,000 for her heroin-trafficking common-law husband - even making money pickups on Bronx street corners for his far-flung operations, which reached into the Dominican Republic, federal documents show.

Balbuena at one point told her boyfriend he "needed to change his lifestyle and live an honest life with a normal job . . . [But] he enjoyed his luxurious lifestyle and expensive cars," the documents say.

Balbuena and her boyfriend, who have two children together, limited their bank deposits to under $10,000 to avoid triggering bank reports, the documents show. They held eight accounts.

Days before her arrest, narcotics cop Jerry Bowen admitted stealing drugs from dealers and giving them to informants as part of a deal to cooperate in a case against three Brooklyn cops nabbed with him last June.

Other high-profile arrests last year:

* Sgt. Roosevelt Green was caught on a wiretap running license-plate checks in his police car for reputed Long Island crack dealer Frank "Big Banana" Wilson, who was correctly concerned that he was being tailed by the feds.

Green, an 11-year veteran, allegedly sold his badge for a pair of sneakers.

Green was indicted by the feds for helping out the drug dealer from his hometown of Wyandanch, LI.

* Jorge Arbaje-Diaz, a Bronx cop for three years, was charged federally with participating in a violent stickup crew that posed as cops while committing 100 armed robberies of drug traffickers along the East Coast, netting millions in cash and drugs, according to prosecutors.

He is suspected of committing the crimes virtually since the day he raised his right hand and swore to uphold the law, prosecutors said.

* Detective Luis Batista, implicated in a cocaine operation, enlisted fellow cop Sgt. Henry Conde to check Internal Affairs Bureau databases to confirm Batista was under investigation. Both were charged federally.

The issue has been of increasing concern as the force has gotten younger and starting salaries have fallen. Annual starting pay dipped to $25,100 a few years ago and has only recently been restored to $40,000.

"You pay a low salary, and people come from neighborhoods where they grew up with people who didn't make the move" in the right direction, one source said.

Not all the wrongdoing uncovered rises to the level of a crime.

Accusations of cops consorting with criminals are extraordinarily difficult to lodge because investigators must prove an individual was aware of someone's unsavory background. And many cops wind up being warned to avoid their errant relations.

But the NYPD trial rooms are littered with cases in which cops are convicted of hanging out with "known criminals," coupled with other departmental violations.

The NYPD declined to comment, but sources said the problem of cops associating with criminals was "a thread running through a lot of administrative cases."

In a bid to analyze past cases, The Post requested NYPD records normally available for inspection at their public information office.

NYPD officials refused to provide them.

Despite the hurdle, The Post examined a nine-month sampling, documenting a dozen department convictions - including a Manhattan lieutenant forced to retire after threatening to beat up a girlfriend and a Bronx cop who lost a month's vacation for disclosing records to a criminal crony.

The NYPD poster boys for consorting with criminals are ex-Detectives Louis Eppolito, who boasted of having a mob capo for an uncle and penned an autobiography, "Mafia Cop," and Steven Caracappa.

They were unmasked as being on the mob's payroll, pocketing bribes and participating in six murders - all while on the force.

"A lot of guys have hard times cutting ties and saying no when someone invariably asks for a favor, a background or warrants check, or help making bail or expediting the system. That's where it starts," a source said.

There's one brand of consorting that's impossible to stop, however.

"How can you monitor who someone falls in love with?" one official asked