The NYPD has told cops how to enforce the state’s new marijuana law — which includes letting adults light up in public, ignoring the exchange of weed unless money is involved and banning vehicle searches based on smell alone.

A four-page memo also says cops can’t “approach, stop or detain” a parolee for smoking or possessing pot, even if they know the ex-con isn’t supposed to be getting high.

Instead, they “should notify the relevant parole officers” to enforce the conditions under which the jailbird was sprung from prison.

The memo says the “sweeping changes” in enforcement are the result of Gov. Andrew Cuomo’s signing of a bill that legalized the recreational use, sale and growing of weed.

It was issued just hours after Cuomo signed the measure into law Wednesday morning.

Specific guidance in the memo tells cops that people 21 and older can legally smoke weed “almost anywhere that cigarette smoking is allowed including on sidewalks, on front stoops and other public places.”

It notes that blazing away “in any of these locations is not a basis for an approach, stop, summons, arrest or search.”

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The memo also says that people can’t be charged with selling pot “unless they receive compensation” and that a “hand-to-hand exchange of lawful amounts … without compensation to a person 21 or over, is not considered a sale.”

In addition, the odor of both “burnt and unburnt” weed “alone no longer establishes probable cause of a crime to search a vehicle,” the memo says.

If a driver appears wasted and reeks of weed or admits “having smoked recently,” cops can search the vehicle’s passenger compartment, but “the trunk may not be searched unless the officer develops separate probable cause to believe the trunk contains evidence of a crime (e.g. gun recovered from under driver seat).”

An NYPD source said the lenient new drug law could prove disastrous for public safety.”We always say, ‘Drugs equal guns.’ When you smelled weed, you could pull a car over. Now, you can’t pull them over,” the cop said.

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“That’s bad, especially with all the gun violence going on.”

Some provisions of the new law went into effect immediately, including those that allow people 21 and older to smoke and possess up to three ounces of pot in public.

Cops on the force will not be allowed to toke up themselves, however, the NYPD said in a statement.

“Rules governing the use of marijuana for members of the NYPD have not changed,” they said.

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Others that will regulate sales at licensed dispensaries and let adults grow as many as 12 pot plants per household and keep a five-pound stash at home are set to be phased in later.

Five pounds of weed is enough to roll more than 3,330 joints, based on data from a 2010 study, “Quantification and Comparison of Marijuana Smoking Practices: Blunts, Joints, and Pipes,” that was published in the medical journal Drug and Alcohol Dependence.

Additional reporting by Bernadette Hogan.

This report originally appeared in the New York Post.