Fifty singers leave Washington this morning to perform the first all-black "Porgy and Bess" ever seen in Australia.

"We hope to knock Australia on its ear," says Randolph Mauldin, who will conduct the performances.

"There was an all-Maori 'Porgy and Bess' down there about 15 years ago," says Roman Terleckyj, stage director for the company, "but that isn't at all the same, is it?"

The cast for this intercontinental "Porgy," working under the auspices of the Spoleto Festival, was assembled in this country for one simple reason: supply and demand. The show has a few nonsinging white roles that will be filled by Australians, and there is no shortage of white actors in Melbourne or Sydney. But for the singing roles, the company has to take along everyone it will need. That means two alternating singers plus an understudy for each of the principal roles: Porgy, Bess, Sportin' Life, Crown and Serena.

In Australia, if an emergency came up, none of these roles could be filled with a last-minute phone call. That's a major difference between Australia and the United States, where the supply of talented, operatically trained black singers is abundant. Down there, it's nonexistent.

"We held auditions in New York and Washington," Mauldin says. "We heard 427 people in four days for 50 jobs, and we turned down another 200 unheard on the basis of their re'sume's or bad referrals."

The singers who emerged from this sifting are a highly qualified lot. There were still rough spots but no serious problems Wednesday night in the final complete run-through in a Kennedy Center rehearsal room. Mauldin conducted the singers informally, using a ball point pen rather than a baton and occasionally pounding out some percussion effects on the top of a piano. He seemed to have every nuance of the score well in hand, including some traditional cuts that he is restoring. "I'm really prepared for this assignment," he says. "I've conducted 'Porgy and Bess' 50 times in the last 12 years, and at last I feel I really know it."

In rehearsal, this "Porgy" seemed to be in remarkably good shape for a show that is not opening (for previews in Melbourne) until Sept. 9. The singers were working together smoothly for the most part, and the solos were impressive. Andrew Smith's rich baritone had an easy, confident swing in "I Got Plenty o' Nuttin'." When Roberta Laws sang "I Loves You, Porgy," even in the harsh lighting of a big rehearsal studio, with a cast in nondescript clothing rather than costumes, with no scenery and a piano rather than an orchestra, the music cast a spell.

As Sportin' Life, a role he has sung at the Metropolitan Opera and will sing next year at the Deutsche Oper in Berlin, Washington baritone Charles Williams was evil incarnate and smooth to the point of oiliness. He moved with pantherlike grace through "It Ain't Necessarily So" -- flirting, dancing solo or with a variety of partners, making explicit passes at some women and implicit passes at all. His body language was as eloquent as his voice -- a result, he confided later, of his studying dance and mime as well as singing "to learn the ABCs of my body."

Williams talked about the role of Sportin' Life as a technical challenge: "Being a baritone in a tenor role can be scary." But in fact, his voice is perfectly controlled throughout the tenor range, up to high D. "I can get up there," he says, "but I couldn't live there." Like all of the principal singers in this production, he is thinking of roles outside the "Porgy" stereotype. For most leading members of the cast, "Porgy" is like Williams' high D: They love it and are proud of what they can do with it, but they wouldn't want to stay there permanently.

Smith has sung the title role once before and dreams about singing it some day at the Met, where he is already scheduled to sing Tonio in "I Pagliacci" and Amonasro in "Aida." But eventually, he says, he wants to drop it from his active repertoire and cultivate his roles in Italian opera -- roles like the Count in "Il Trovatore," Scarpia in "Tosca" and Rigoletto, all of which he will be singing next season.

"I have seen some of my colleagues get locked into 'Porgy,' trapped in their roles and unable to get other roles although they are very good singers," he says. "I don't want that to happen to me."

Still, he loves the role -- and even more he loves that of the villain Crown, which he sang for 17 months with the Houston Grand Opera. In the same way, Laws loves the role of Bess but does not care to be stereotyped. She remembers the first operatic role she ever sang, the Countess in "The Marriage of Figaro," and looks forward to singing Mimi in "La Bohe`me."

For Williams, the chief unfulfilled ambition is the role of Papageno in "The Magic Flute," and he has just agreed to do it with the Prince George's Civic Opera. This, he hopes, will be a first step, not necessarily toward singing it at the Metropolitan Opera but at least singing it for the Met's artistic director, James Levine. "That is one of my dreams," he says. "I love that role, and it's me. I want to sing it for Levine and hear what he will say; he is so kind, so encouraging, and a very good teacher." Meanwhile, he is looking forward to his Kennedy Center debut in a recital in October and to yet another new role, El Gallo in "The Fantasticks," next December for the American Showcase Theater Company in Alexandria. "I like the role," he says, "but even more I like the company. This will have a completely mixed cast, with no attention to color. You may see a white father with a black daughter."

Many admirers of Washington baritone Gordon Hawkins have been waiting for his debut in the role of Porgy. As it turns out, they will have to fly to Australia if they want to witness that event. "If anyone is willing to buy the airplane ticket, I'll get them a free ticket to the show," Hawkins says. He is cast in the supporting role of Jake but covering that of Porgy, and he expects to do at least one matinee performance, perhaps as many as three, during "Porgy's" three weeks in Melbourne and two in Sydney. His career has reached the point where he has to turn down a significant number of engagements, and he finds that pleasant.

"I've always wondered," he says, "what it would be like to be very busy and get the opportunity to say, 'No, I'm sorry, I can't do that; I'm busy with something else.' Not to be impolite or give people a hard time; just to know that you are in demand. Now I'm reaching that point, and it's good."

Amonghis many engagements in the upcoming season will be a role in the Washington Opera's "L'Italiana in Algeri." This is only one of many links between this "Porgy" and the Washington Opera, which, institutionally, has nothing to do with the production. Mauldin is the Washington Opera's music administrator and will conduct the company's new "Ruddigore" next season. Stage director Terleckyj is the Washington Opera's artistic administrator and will assist Gian Carlo Menotti as stage director next season in the new production of Menotti's "The Consul."

Perhaps the most familiar touch of all is the fact that the sets for this "Porgy" are designed by Zack Brown, whose work for the Washington Opera establishes standards in the field. The sets are being built in Australia, but paintings available here show Brown's usual meticulous workmanship and a fine familiarity with the atmosphere of Charleston, S.C.'s, Catfish Row, where "Porgy" takes place.

The Spoleto Festival in Charleston might seem a logical venue for the production after its Australian run is ended, but Terleckyj is not sure. "The idea behind the festival is cultural exchange," he says. "Bringing 'Porgy and Bess' to Charleston doesn't sound like cultural exchange." But he may have been reluctant to discuss plans that are not completely worked out or not ready to be announced. The Spoleto Festival embodies more than one idea, though cultural exchange is a large part of it. Backstage rumor has also held for some time that this production, which has so many Washington elements, may eventually be brought to Washington, where it was rehearsed but not played for the public. Terleckyj does remark casually, "I think the American public is beginning to realize you can't get enough of 'Porgy.' "

The subject changes to Terleckyj's concept of the opera. "A few days ago," he says, "an Australian reporter asked me, 'Will it help us to understand black people better?' Well, that would be nice, but I don't think it is what opera is for. This opera is a love story, stripped down to the bare essentials. The driving force is Bess' need to be loved and the transformation in her life when Porgy begins to love her. I'm not trying to make this 'Porgy' different; I don't want to be controversial with 'Porgy and Bess.' But I do want to make it fresh; we want to do it again as if from the beginning."