The Reagan administration, in a decision of major significance for its policies in southern Africa, has decided to pick up the threads of the Carter administration's program of international negotiations to bring independence to Namibia.

The administration's decision implies a more conventional and moderate course in the region than many had expected.At the same time, its likely position in those negotiations, as projected by President Reagan in an interview Friday with The Washington Post, suggests a delay in a Namibia solution and is likely to be unpopular in Africa.

The administration's decision to continue U.S. efforts toward an internationally negotiated Namibian settlement was conveyed to Nigerian Foreign Minister Ishaya Audu by Secretary of State Alexander M. Haig Jr. at the State Department Friday, according to administration sources.

Haig's disclosure is believed to be among the factors contributing to the optimism about U.S. policy in southern Africa expressed by Audu, the secretary of state's first official visitor from Africa, after the meeting.

The African visitor was told that the new administration has completed the initial phase of its far-ranging review of policy toward the continent. In this connection, several recent decisions have been made to reduce the soaring apprehensions of black African states:

In a move related to the Namibian decision, the State Department persuaded Angolan guerrilla leader Jonas Savimbi was to have arrived in New York today for a visit that would have heightened speculation about the renewal of U.S. aid to the rebels fighting the Cuban-backed Angolan government.

Vice President Bush has been named to lead a U.S. delegation to an international conference April 9 and 10 on the "urgent and growing problems" of African refugees. The high-visibility U.S. participation in the conference, which is sponsored in part by the Organization of African Unity, imples a major U.S. donation to efforts to aid the refugees.

The Reagan administration, despite its budget-cutting emphasis, pledged last week to contribute $225 million over three years to start-up aid to Zimbabwe, where British-led negotiations produced black majority rule through elections. This U.S. contribution, next only to that of the World Bank and Britain, is aimed at improving Zimbabwe's chances for stability and aligning it to the West rather than the East.

All these are quite different signals from those that rang alarm bells in African and European capitals in recent weeks. The previous signals suggested that a preference for white-ruled South Africa over black Africa and for military confrontation over negotiations would characterize Reagan's policy toward the continent.

Reagan, in the Post interview, spoke of "a continued friendship" with South Africa despite that country's "repugnant" policy of apartheid, but he laid most stress on continued friendship with "the emerging African states, the black African states."

Saying that some of the African countries "have a chip on their shoulder toward us," Reagan said the United States is "going to take steps" to bring about better understanding with them.

Reagan volunteered in the interview that "we want to see a peaceful solution to the Namibian situation." Reagan added that, in his view, the solution involves an election that "should follow the adoption of a constitution that guarantees equal rights to all people in that country -- property rights, minority rights."

Namibia, a large and mineral-rich former German colony on the west coast of southern Africa, has been run by South Africa since World War I. Its future is bitterly contested between South African-backed local groups and a black liberation organization, SWAPO, based in neighboring Angola.

A five-power international "contact group" composed of the United States, Britain, Canada, France and West Germany negotiated over the span of three years a plan for United Nations-supervised elections in Namibia. This plan received a grave setback when South Africa and the leading internal party, the Democratic Turnhalle Alliance, rejected it this January.

The reagan administration has been under heavy pressure from allies, especially Britain and Germany, to continue the international effort for a negotiated settlement in Namibia. A great deal of pressure and concern also has come from black African states, which are sponsoring a U.N. Security Council debate on South Africa's role in Namibia next month and may at that session seek mandatory sanctions, courting a veto from the United States.

Dirk Mudge, the leader of the Democratic Turnhalle Alliance, who visited here earlier this month, advocated constitutional guarantees for whites and property owners along the lines mentioned by Reagan. Agreement on such a constitution before the proposed U.N.-sponsored election in Namibia would require new and probably extensive negotiations.

In resuming a U.S. drive for a negotiated settlement, according to official sources, the administration had decided to launch high-level consultations with African parties. Senior officials, including Chester A. Crocker, who has been nominated to be assistant secretary of state for African affairs, are expected to visit the region for this purpose soon.

Generating much concern in Africa is the Reagan proposal that Congress repeal the Clark amendment forbidding covert or overt U.S. aid to rebel forces in Angola. Any such aid policy would seriously complicate a negotiated Namibian settlement, because of the key role of the Angolan government in security aspects of the Namibian negotiations.

Savimbi, the leader of the UNITA guerrilla faction formerly supported by the CIA, had planned a U.S. visit starting early this month. The Reagan administration advised him to delay while it sent am emissary to confer with him secretly in Morocco two weeks ago. Now the State Department, reportedly with White House approval, has prevailed on him to delay his visit again to avoid a rekindling of speculation about aid to his cause, sources said.