The hard-nosed assessment of the experts is that economic sanctions against South Africa, standing alone, are unlikely to divert the present repressive government from its commitment to maintain apartheid.

Let's concede the point: History shows that economic sanctions, except against tiny and powerless countries, haven't been too successful. Often, the country imposing sanctions shoots itself or its business community in the foot.

But something more appears to be at work in this case, thanks to the courageous rebellion of the oppressed blacks: Panic in the international banking community has resulted in a collapse of the South African rand from a range of 60 to 70 cents to about 35 cents. Capital is flooding out of South Africa, and the government has been forced to close trading on the foreign exchange and stock markets.

The forced devaluation of the rand will impose a huge penalty on South Africans in the form of price increases for imports and leave the South African trade balance in wreckage: At the 60- to 70-cent level, import-export trade was about in balance. If the present situation continues, the South African international balances will begin to show large deficits.

In all probability, the South African economy will continue to be battered by events. The black workers who man the nation's coal and gold mines are planning selective strikes, and ultimately may pull off the first general strike in South African history.

Meanwhile, banks throughout Europe have been calling in their loans to South Africa, or tightening up their lines of credit, as a response to President P. W. Botha's refusal to make substantial changes in the apartheid laws.

As a result, Gerhard de Kock, governor of the South Africa Reserve Bank, is on a hasty mission to Europe, conferring with central bankers in London and Zurich on rescheduling South Africa's $17 billion debt. Two-thirds of that debt matures within a year, which may force the government to ask for a moratorium that would hurt its credit rating for years to come.

Thus, President Reagan will be making a monumental mistake if he vetoes a sanctions bill, using the excuse that sanctions alone won't force changes from the Botha government.

Two experts in the field of economic sanctions, Gary Hufbauer and Jeffrey Schott of the Institute for International Economics, estimate that the relatively mild retaliatory measures now being considered by Congress would add up to no more than a $10 nick in the $2,500 per capita income in South Africa.

But given the way the South African economy is reeling under pressure from the banks, economic sanctions -- which by themselves might be flicked off by Pretoria as a mere annoyance -- become a reinforcement for the more important internal pressures for change, Hufbauer agreed in an interview.

Even if it appeared that economic sanctions would accomplish little, there is a principle involved. Sometimes, one has to do what is right, even if it's only a cry in the wilderness. So long as human beings are being killed and so long as those appalling pictures of armed South African police clubbing helpless blacks jump out of our TV tubes, no one should oppose any effort to inflict pain on the Botha government.

There are some doubters. Columnist George F. Will, in a recent column scoffing at the proposed use of sanctions, asked: "Are sanctions to remain in place until Pretoria changes policies? If so, which policies?"

Hufbauer and Schott, in an article written for the Financial Times, have an answer. They suggest that coordinated pressure on South Africa by European countries and the United States should set an achievable target. No one expects that sanctions will wipe out apartheid overnight.

Together, the key countries would demand specific and pragmatic reforms over a period of time that would erode the major underpinnings of apartheid.

Thus, Pretoria would be asked initially to endorse, and later to implement, the program advocated last January by six South African employer groups: universal citizenship, meaningful political participation for all blacks, free and independent trade unions, the right to own shops and conduct business anywhere in the country, and an end to forced relocation of people.

That is the minimum the West can ask for and keep any of its self-respect. There would still be a long way to go before South African residents attain full civil liberties with complete equality among the races.

Pretoria will resist: There will be no easy abandonment of the racial bigotry that the minority of whites employs to keep the black majority in slavery. But the pressure will begin to hurt -- in Pretoria's image of itself and in the pocketbooks of the whites.

The critical element is coordination. France and Sweden already have banned all new investment in South Africa (Sweden has done so since 1979). But the key players are England (the biggest investor in South Africa) and Germany, which together have 50 percent of all foreign investment in South Africa. Neither wants to pull out.

So far, the three leading conservative politicians of the Western world -- Ronald Reagan, Margaret Thatcher and Helmut Kohl -- have put business ahead of morality: They condemn apartheid and the repressive climate in South Africa, but resist the move toward sanctions or official approval of financial divestment. This is the time for business and financial communities in all three countries to lead the way for the politicians.