A former lobbyist for the South African government who is now general counsel in the White House trade representative's office left for South Africa on an official mission yesterday, even as the Reagan administration was announcing a relaxation of restrictions on trade with the Pretoria government.

Donald deKieffer, reached by telephone in Geneva, said there was no connection between his trip, during which he will address American businessmen and meet with high-ranking South African trade officials, and the new administration regulations.

He said that he will discuss "general U.S. trade policy" and that he has made clear to the South Africans that he cannot discuss U.S.-South African trade issues or conduct any negotiations on such issues because of his previous role as a lobbyist.

Although deKieffer said he has been careful to "recuse," or disqualify, himself from any discussion of trade with South Africa, he acknowledged in an earlier telephone interview that he attended a meeting of high-ranking officials from the Commerce and State departments Jan. 5 at which South African issues were on the agenda.

Officials who attended that meeting have said that although other issues were discussed, South Africa was a major component of that session.

These officials said deKieffer did not play a major role in the meeting.

DeKieffer said he was "very sensitive" to suggestions of potential conflict of interest, which is why he submitted a statement to William E. Brock, the White House trade representative, in March 1981--after appointment as general counsel to the U.S. trade representative's office.

DeKieffer's statement, which he provided to The Washington Post, included a list of his former clients while a lobbyist and said he disqualified himself from any discussion of matters that would affect his former clients directly.

"It's like Jimmy Carter speaking to an agricultural group," deKieffer said of his meeting with American businessmen in South Africa. "I know he's recused himself from talking to the peanut industry since he is a peanut farmer . . . . But I fully intend to talk to any group that's qualified to talk on general trade policy."

DeKieffer, whose law firm spent tens of thousands of dollars on behalf of South Africa while he was a registered lobbyist for the Pretoria government until 1979, will arrive in Pretoria Saturday and stay until Thursday. Among the high-ranking officials from the ministry of commerce, trade and industry he will see are Trade Minister Dawie De Villiers; the director general of the ministry, Tjaart duPlessis, and the deputy director, Frans Scheepers.

While a lobbyist, deKieffer regularly filed reports with the Justice Department as required under the Foreign Agents Registration Act. In one six-month period, he spent as much as $17,847 on entertainment expenses in connection with his lobbying efforts, according to his statements.

He ended his association with South Africa after reports were made public of a clandestinely funded campaign by the South African Information Department aimed at improving South Africa's image abroad, particularly in the United States. DeKieffer has never been linked to any wrongdoing in connection with that campaign, but his law firm's fees were paid by the Information Department.

DeKieffer said he was invited to speak to the American Chamber of Commerce in Pretoria and that he would talk about general U.S. trade policy, as he did at several similar stops in Europe, including London.

"I have already talked to the State Department and told them I can't negotiate with them the South Africans on trade matters and that I will have somebody with me when I meet them," deKieffer said. "I have recused myself from any negotiations."

A White House spokesman said deKieffer's trip was cleared through the State Department and that a memorandum was sent to presidential counsel Fred F. Fielding.

A spokesman for the South African Embassy in Washington said that after the embassy learned of deKieffer's intention to visit South Africa, it suggested to Pretoria the names of several officials he might meet with, but that the embassy did not arrange any other details of the trip.

U.S. envoys in Johannesburg and Pretoria said the visit was "unheralded" and expressed some irritation at not having been advised of the trip in advance. DeKieffer is also to speak to the Cape Town Press Club Wednesday.

Several U.S. officials said that while there is no evidence that deKieffer has done anything wrong by going to South Africa or attending the January meeting, they questioned whether a person who was once a lobbyist for a government should associate in any way with matters involving that country. DeKieffer, however, insisted that he has "hewed the line" and been careful to be public about his potential conflicts.