New Zealand Prime Minister David Lange threatened today to reduce South Pacific maritime surveillance and other activities important to the United States in retaliation for U.S. sanctions against his nation, which has refused to allow nuclear ships to visit there.

The recently installed Labor Party leader, speaking to American businessmen here, raised the dispute over New Zealand's ban on U.S. nuclear warships to a new level. He said the United States informed him this morning that it had "drastically scaled down" cooperation with New Zealand -- primarily in intelligence sharing and defense -- and he suggested this would only hurt the United States.

"We have military assistance programs with South Pacific island states. We have the prime responsibility for maritime surveillance of the vast South Pacific. We have a force stationed in Singapore," Lange said.

"If the United States diminishes defense cooperation under ANZUS, this will in turn diminish our capacity to go on playing a role in Southeast Asia and the South Pacific," he said, calling New Zealand's role "a contribution to the safeguarding of United States and Western security as a whole." ANZUS is the Australia-New Zealand-United States defense pact.

Lange said William A. Brown, U.S. deputy assistant secretary of state, told him at a one-hour meeting this morning that Washington was canceling joint military exercises with New Zealand and cutting off intelligence "of the raw, military sort." In opening remarks added at the last minute to his luncheon speech here, Lange called the U.S. actions "a dramatic scaling down of cooperation" that is "serious and . . . to a degree damaging."

"They are not, in my view, the kind of actions which a great power should take against a small, loyal ally which has stood by it, through thick and thin, in war and peace," he said.

Lange made no effort to soften his government's ban on any nuclear-powered or nuclear-armed vessels calling at New Zealand ports. Widespread antinuclear sentiment among New Zealand's 3 million people helped Lange's party return to power last July.

"My point is that the security of the South Pacific does not depend on the occasional deployment of nuclear weapons in New Zealand ports, and in making an issue of it, the U.S. gives scant regard to its own long-term interests in the region," he told a California business group called the N Z Connection.

Brown said after his morning meeting with the prime minister that their talks had been "frank and candid." He indicated that administration officials felt that the effective ban on all U.S. warships, since policy bars disclosure of which carry nuclear weapons, will hinder U.S. ability to meet any threat in the Pacific.

Although Lange is flying directly to a conference in London after this brief, unofficial visit, Brown said the New Zealand deputy foreign secretary would go to Washington Wednesday for more talks. U.S. and New Zealand officials have indicated that they do not expect to resolve their differences.

Lange said the United States appears bent on curtailing the defense relationship "until such time as a government is elected in New Zealand which will admit American nuclear weapons," a view echoed by Reagan administration officials. He also said he thought the administration sought to warn other allies against a similar prohibition on visits by nuclear ships.

Lange told reporters after the speech that his policy is designed to prevent deaths from nuclear accidents and reduce the likelihood of Soviet nuclear attack, but is principally justified by a state's right to refrain from arming itself with nuclear weapons when not threatened by such weapons.