Johannesburg — With a massive mandate from whites, South African Prime Minister Pieter W. Botha is expected to quicken the tempo but not alter the substance of his policies of so-called ''reform.''
This means ''reform'' will continue to be within the parameters of apartheid. But there will be adaptations with the intent of making the exclusion of the black majority from central government more palatable and sustainable, say informed observers here.
''There is no hidden agenda,'' Mr. Botha emphasized immediately after winning with a huge majority a white referendum on his plans to bring Coloreds (persons of mixed-race descent) and Indians into Parliament.
Blacks remain excluded in the new Constitution. It is government policy to relegate them politically, and as much as possible, physically, to tribal ''homelands.''
But the government concedes that urban blacks who live outside the homelands represent a major anomaly. The government recognizes these blacks are now ''permanent'' and essential to the country's economy.
The government's next major ''reform'' initiative is widely expected to deal with the urban black population. But by all accounts the government's aim is to accommodate somehow these blacks in the ''homelands'' framework rather than fundamentally alter the ''homelands'' policy.
On the heels of the referendum vote Botha convened a special Cabinet committee set up some months ago to consider the constitutional future of blacks living outside the homelands.
The government's blueprint for the homelands appears to be the formation of some confederal structure that would permit the black tribal authorities and the white government to confer on certain matters, without jeopardizing white control over the bulk of South Africa. The Cabinet committee will be considering how to fit the urban black population into this structure.
Most blacks reject the government's approach to the issue. The homelands are not regarded by blacks as legitimate substitutes for political power in South Africa. And the newest distinction of dividing blacks into ''urbanized'' and ''homeland'' categories is seen as artificial and unjust.
Nearly 40 percent of South Africa's 21 million blacks are urbanized. For some time the government has been hammering out a policy to stabilize the urban black population by granting it certain privileges not enjoyed by the majority of blacks, and at the same time firmly regulating its size through strict influx control measures.
The mix of government policy regarding urban blacks suggests a willingness to make limited concessions of political power as long as the size and distribution of this population group is regulated so it never becomes a threat to whites.
One recent example was a landmark court victory won by black laborer Meholo Rikhoto. The decision established that blacks who had worked continuously for one employer for 10 years were entitled to ''permanent'' rights to live in the city. One of the main benefits of permanent city rights is that worker's family is allowed to come live with him in the city.
The government vowed it would accept the decision, a stance typifying what some saw as Pretoria's new ''reformist'' attitude. But the government has since amended a law that makes it extremely difficult for the families of men like Mr. Rikhoto to come to the city.
New applicants for the rights won by Rikhoto must have ''approved'' housing before their families can join them. And experts point out that in Soweto, for instance, the waiting list for black housing goes back to 1969.
In the next few months the carrot and stick nature of the government's policy toward urban blacks will be apparent.
At the end of this month a round of new local elections will be held in black townships. Existing black government bodies have so little real power that most blacks haven't bothered to vote. But new legislation has given local government bodies more power. Polling will be closely watched as an indicator of whether the new structures have any credibility with blacks.
At the same time, the government has promised to seek passage next year of a new law that will overhaul influx control, reducing legally permissable urbanization ''to the point of extinction,'' notes an analysis by the Black Sash human rights organization. And blacks who come to the city illegally, as well as those who in any way help them, will be dealt with more harshly.
In the short term, the government will be preoccupied with implementing its new Constitution. The government had made vague promises about testing Colored and Indian opinion. But there is growing speculation this ''test'' might be an election for Colored and Indian members of Parliament.
Whatever, analysts expect the government to move forward with so-called ''reforms'' for urban blacks to entice more skeptical Coloreds and Indians into the new constitutional set up.