SHEENA DUNCAN is that rare creature, a reformer who is great fun. With a smoky, contralto voice and a rich, ready laugh, she goes about her work of trying to end apartheid in her native South Africa with zest and endless confidence that it is all going to come out all right. She is president of the Black Sash, a political pressure group made up of white women.

She was born in Johannesburg in 1932, the eldest of five children in a well-to-do Scottish family. She had a white nanny and went to a private school.

"It was a home where conversation was conversation, not gossip," she says. "But the focus was on World War II, and "when we talked about injustice, we were thinking about the Jewish people and that kind of thing."

At her school, Roedean, she heard such speakers as Trevor Huddleston, the Anglican clergyman who lived in a black township, and Alan Paton, the author of "Cry the Beloved Country." After graduation, she was carefully escorted by her mother to Edinburgh and delivered into the hands of her aunts for further education as a teacher of domestic science.

When she went back to Johannesburg, she married the boy next door, an architect, and they went off to Rhodesia, where her three children were born.

Her mother, meanwhile, founded the Black Sash in l955, specifically to protest the removal of blacks from the voting rolls.

"She lost most of her friends," says Sheena Duncan. "She was expected to go to the club and play golf or tennis and to do some active welfare work as well. But she and the four women who also started it could see that the men were just dithering around and that they had to go to the roots of the problem , which is apartheid."

The Black Sash, which now has 1,500 members, does educational and paralegal work for victims of apartheid. Before political assemblies were banned, the members put on their black sashes and marched in demonstrations. Now they have to stand alone, with head bowed, holding a sign.

"Gets quite lonely," says Duncan.

While her children were small, Duncan spent one morning a week producing written material for the Black Sash. Now she spends full time, with the "entire support of my husband, who puts up with a great deal," including absences caused by foreign speaking tours like the one that brought her to Washington.

She's gotten used to being called a "communist" and a "kaffir- boetie," which is Afrikaans for "nigger-lover." Like other South African activists, she has her telephone tapped and her mail opened.

"We are protected because we are white and because we are female -- they're terribly chauvinist. The government has never regarded us as a real threat. The thing is that the black people have suffered terribly. You don't even get embarked if you are easily intimidated."

She will return to South Africa Monday. She thinks the situation is totally different from what it was.

"The big difference is the deeply rooted mobilization in the black community. In 1960, after the Sharpeville shooting, the government banned the Pan African organization and the African National Congress, and there was no political movement at all. Today, the resistance is deep in the black communities. They are not giving way this time -- they think it is the beginning of the end of the struggle."

She has enjoyed her American tour, except for a moment in Chicago when a man asked her if she thought South African blacks were "able to exercise a responsible and informed vote." She asked him tartly in reply if the American electorate were capable of it.

It has been "a great comfort" to find so many Americans in disagreement with their government about "constructive engagement," a policy she regards as a disaster.

"When we were having a campaign for a white referendum to vote no on the new constitution, which calls for race classification, your ambassador was welcoming it as a step in the right direction. What worries me is the distorted analysis of the world that's behind it, that you're either communist or anticommunist."

She is all for trade sanctions and disinvestment, because "for the first time the businessmen are putting pressure on our government, calling for quite fundamental changes in our society."

She is hopeful that with her great friend Bishop Tutu leading the way, her beloved country can find its way to a peaceful solution. And if it can't, if there is a bloody civil war and people like herself lose everything?

Says Sheena Duncan cheerfully, "It would be a price worth paying if we were to reach justice and democracy. One has to be prepared for a change in lifestyle. And I have great confidence. It's astonishing how little revenge has been sought in most of Africa; there has been very little rage against white people."