THE REAGAN administration smelt a whiff of peacenik smoke from Down Under, immediately yelled fire and began treating the prime minister of New Zealand like an arsonist.

Prime Minister David Lange was merely keeping a campaign promise, but from the thunders of Washington, you might have thought he had declared that his country had gone neutralist, or even become a second Nicaragua.

All he said was that U.S. warships that were either powered by nukes or armed with them would not be welcome in nuclear-free New Zealand, thereby redeeming a pledge made in his campaign.

Faced with this practical move from one of our smaller and stauncher allies, the Reagan administration could have looked the other way. That's what Japan, which is also nuclear free, does when American warships, which may or may or may not be armed with nuclear weapons -- it is our policy never to say -- come to call. That Japanese do not ask.

But the administration is in a fire- breathing phase, and reacts like a choleric giant to any challenges to its authority. Lange was let to know that Ronald Reagan is gravely put out and that New Zealand's standing in the ANZUS alliance may be in doubt. Joint military exercises in the area have been canceled. Once-shared intelligence reports have been snatched out of Wellington's hands. There are rumbles, although not from officialdom, about trade restrictions.

Making a heavy out of New Zealand is of course, extremely heavy work.

Its rugged, matey inhabitants could never, as a host of World War II vets would vehemently assert, be mistaken for pacifist wimps or fellow travelers. Lange, 42, a Methodist minister's son, and a gifted lay preacher, heads a Labor government, with economic policies much like Reagan's.

Of New Zealand's four political parties, three are robustly in support of keeping the sparsely-inhabited country a "nuclear-free zone." And before the administration goes further in fostering the unfortunate fallout notion that to be antinuclear is to be anti-American, it should note the newest poll, which shows that although 60 percent of New Zealand's 3 million people favor Lange's stand on banning the warships, 70 percent favor staying in the ANZUS.

So why make a federal case out of a practical political necessity undertaken by the head of a sovereign country?

Secretary of State George Shultz said last summer that U.S. warship visits are "essential" to New Zealand, but the facts are contrary. According to the authorities in Wellington, there have been only 11 U.S. ship visits between 1960 and 1983.

The real reason for trying to bully New Zealand back into line is that Lange is rocking Reagan's nuclear buildup dreamboat, and his heresy could be catching.

Lange has taken the debate back to ground zero at a moment when the world was lost in the stars, contemplating Reagan's Strategic Defense Initiative, the greatest escalation since the MIRVing of missiles. He has defied the right of the United States to dominate nuclear policy making. He has reminded the world that nuclear weapons are dangerous. He has also suggested to small nations that they can take their destinies into their own hands. During the campaign, he constantly harped on the theme that there is no defense in a nuclear war: U.S. weapons fired in defense of New Zealand would wipe it out.

It comes at an awkward time for Reagan. Three years ago, his nuclear buildup was menaced by the nuclear freeze. It was a movement that had impeccable sponsorship, scientists, churchmen, and broad acceptance. It spread from New England town meetings to city councils across the country. It seemed irresistible as a political force.

Today, hardly a trace of it remains. Reagan stamped it. He said it was dangerous, warned that it would freeze Russianority, that its adherents were being manipulated by the KGB.

Reagan is coming on as a candidate for the Nobel Peace Prize. Resistance to Star Wars is melting, here and abroad. He has brought the Russians back to the bargaining table. Reducing the number of nuclear weapons is not enough for him; he wants to eliminate them entirely. Meantime, he must have the MX, he must have Star Wars. His people talk about Soviet treaty violations. They set the criteria for verification ever higher.

But people want to believe that Geneva will do something about the nuclear weapons that are piling up on both sides. They have stopped asking how many more on the way to none.

The peace movement in Europe, demoralized by the deployment of the Euromissiles, has been dormant -- or it was until the voice from Down Under was heard.

David Lange is indeed a dangerous man. Unarmed, but dangerous.