JOHANNESBURG, DEC. 29 -- President Pieter W. Botha's plans to postpone whites-only parliamentary elections scheduled for 1989 appeared to unravel today as the Colored, or mixed-race, Labor Party demanded the repeal of South Africa's housing segregation laws as a condition for agreeing to such a delay.

The decision means that Botha's ruling National Party probably will be forced to face a growing challenge from right-wing whites in about a year, which it had hoped to avoid until 1992.

Botha has said he will never agree to completely scrap the 1950 Group Areas Act, which segregates residential areas by race. He has approved limited reforms that could lead to a few racially integrated neighborhoods if the majority of their residents request it.

The Labor Party, which controls the Colored chamber of the segregated tricameral Parliament, voted at its convention in Pretoria today to demand repeal of the Group Areas Act before agreeing to an election postponement.

Under South Africa's Constitution, Botha needs the approval of the Colored chamber and the Indian house of Parliament before going ahead with his plans for a delay.

Whites went to the polls in a general election in May. On Aug. 12, Botha obtained unanimous Cabinet support for changing the Constitution so that each chamber of Parliament would hold office independently for five years. That would place the next national elections for whites in 1992.

The far-right Conservative Party, which won 22 seats in Parliament in the May voting, has protested bitterly that Botha is attempting to rewrite the Constitution in order to put off a confrontation.

The leader of the Labor Party, the Rev. Allan Hendrickse, initially supported Botha's decision, but subsequently threatened to withhold his party's endorsement. That led to a confrontation with the president in August and Hendrickse's resignation from the Cabinet.

South Africa's 26 million blacks are not permitted to vote in parliamentary elections.

Even before last May's whites-only balloting, both the Conservative Party and the antiapartheid Independent Movement were planning strategy for a rematch in 1989, hoping to make inroads into the National Party's 40-year dominance of Parliament.

The previous all-white Parliament was elected in 1981, and white elections would normally have been due by 1986. But the country later adopted a tricameral Parliament, and constitutional revisions in 1984 set new elections in 1989 for all three houses.

Opening his party's convention last night, Hendrickse declared that apartheid is "alive and well" in South Africa, adding, "unless this {Group Areas} legislation is deleted from the statute book, the positive changes which have taken place will remain empty, hollow and meaningless."

The Labor Party, Hendrickse said, would have to decide whether to continue as a participant in the parliamentary system or to work for change from the outside.

The convention voted unanimously to stay in the legislature until 1989, at which time it will decide whether to withdraw.