In assessing a government policy, a good journalist is well advised to turn to advocates familiar with the ins and outs of that policy. It's just as advisable, however, to make sure the advocate knows what he is talking about.

Consider, for example, New York Civil Liberties Union Executive Director Norman Siegel, cited by Howard Kurtz in a Nov. 27 Post story on New York's new program to move from the streets and into treatment programs mentally ill homeless persons at risk in the foreseeable future. The program has received widespread editorial support from local newspapers and national columnists. Relatives of some of those we have assisted have thanked us for helping their sons and daughters, husbands and wives. Most important, judges have ruled that our efforts were appropriate in five of the six petitions for release they have heard. The sixth decision, in the case of Joyce Brown, is on appeal.

Mr. Siegel opposes our program. "Norman Siegel," reported Mr. Kurtz, "said it is no coincidence that {Joyce} Brown is black. He charged that there are 'racial overtones' to {Mayor} Koch's policy, saying that more than 80 percent of the city's homeless are black and that those who have been hospitalized have all been removed from white Manhattan neighborhoods south of Harlem."

But consider what Mr. Siegel ignores. We announced, for example, in advance of the program's start-up, that we would cover all of the West Side and 96th Street on the East Side, as had been the case for the past five years under the program's more limited scope and definition of when we could intervene. We announced, too, that we will consider expansion of our program to the other four boroughs as soon as assistance has been provided to all of the identified caseload in Manhattan. Not a word from Mr. Siegel.

As to his charge of "racial overtones"? Well, since the expanded program began, we have moved 35 mentally ill homeless people from the streets and into a special unit at Bellevue Hospital. Of these, 21 are white, eight are black, five are Hispanic, and one is Asian -- obviously a racially mixed group. That's because our program is based on the condition of a person's mental faculties, not on the color of his skin.

A good advocate -- and, for that matter, a good source -- pays attention to the facts. Regrettably, Mr. Siegel ignores the facts so that he's free to flail against fictions of his own creation.

EDWARD I. KOCH Mayor New York