JOHANNESBURG, South Africa (AP) _ The government Thursday reversed its decision to expel Los Angeles Times correspondent Michael Parks, a rare reprieve for a foreign journalist facing an expulsion order.

Home Affairs Minister Stoffel Botha said he agreed to let Parks remain after getting unspecified ''assurances and undertakings'' from executives of the newspaper.

Los Angeles Times Editor William F. Thomas said, ''The assurances I gave the minister were that the Times will continue to strive for the fair and balanced coverage it promised when the South African government allowed it to open a bureau in Johannesburg 15 years ago.''

Parks, who would have been the sixth foreign correspondent expelled from South Africa in the past year, was told Dec. 9 that his work permit would not be renewed and that he must leave South Africa by the end of December.

Later, Botha extended the deadline until Jan. 31 to allow Thomas and Los Angeles Times Foreign Editor Alvin Shuster to travel to South Africa to appeal the order. Thomas, Shuster and Parks met Botha and other senior government officials in Cape Town on Monday and Tuesday.

Earlier this month, The New York Times was refused permission to make a similar appeal when its correspondent Alan Cowell was told to leave South Africa by Jan. 10. Botha also turned down the visa application by Cowell's designated successor, Serge Schmemann, leaving the New York Times without a reporter in South Africa.

Four other foreign journalists - Newsweek correspondent Richard Manning, CBS television cameraman Wim de Vos, Israeli reporter Dan Sagir and West German TV correspondent Heinrich Buettgen - have been ordered out in the past year. Parks, 43, declined comment after the decision was announced. He has been based in South Africa since mid-1984.

The government sharply tightened press restrictions on both local and foreign journalists Dec. 11, six months after imposing a nationwide state of emergency. The new rules required for the first time that some material be submitted for official approval before publication, including reports on some peaceful protest as well as unrest.

President P.W. Botha said the controls were needed because communist radicals were using the media to foster unrest and revolution in South Africa, where the 24 million blacks have no vote in national affairs.

By law and custom, South Africa's system of apartheid establishes a racially segregated society where the 5 million whites control the economy and maintain separate districts, schools and health services.

In other developments:

-In Durban, U.S. consul general Martin L. Cheshes said the vice consul, Mike Matera, was refused permission Thursday to talk to a detained American missionary in the Transkei black homeland.

Transkei officials in Umtata barred a visit to the Rev. Casimir Paulson, 51, a Roman Catholic missionary from the Marianhill order in Dearborn, Mich., Cheshes said. Transkei is considered independent by South Africa, but it is not recognized abroad.

-The Azanian People's Organization accused a rival anti-apartheid group, the United Democratic Front, of shooting two relatives of its general secretary, killing one of them.