A successful Manhattan investment banker who, at the height of his career, takes over the operation of a heavily indebted theater complex with declining receipts and a leaking roof may not be making the best use of his time and talents.

But that is not the way Australian-born financier James Wolfensohn, 57, named last year as chairman of Washington's Kennedy Center for the Performing Arts, sees the situation.In eighteen months on the job he has wiped out the debts, fixed the roof and begun to move toward his goal of building the center into the cultural flagship of U.S. performing arts.

Among his ambitious plans are commissioning plays, musical works and ballets in cities across the nation and bringing them to Washington for presentation on one of Kennedy's six stages and what he calls its "seventh stage" - cultural television channels.

The Kennedy Center is a hulking, white marble-faced rectangular building erected on a broad sweep of the Potomac River in 1971 to commemorate assassinated President John F. Kennedy. Its builders failed to allow for the correct expansion of its roof in Washington's blistering summer heat and it cracked and leaked.

Immediately on his appointment in March 1990, Wolfensohn threw himself into fund raising. He talked Congress into granting an extra 45 million dollars for restoring the building and to pay off debts. He persuaded corporate executives to help plug the Center's annual operating deficit.

Already the first of his production plans are getting under way. In June as part of his extensive plans for state festivals, Texas sent no fewer than 30 theater, dance and musical companies to perform on Kennedy's stages.

Next year the Kennedy's National Symphony Orchestra will begin a "Tour America" program spending a week in each city, performing, giving master classes and bringing students in from local music schools, some of whom may be selected to come to Washington to perform.

"We have lagged so far on theater mainly because of my own lack of expertise," Wolfensohn says. He has hired Lawrence Wilker, head of a Cleveland, Ohio, performing arts complex - "He lives and breathes the theater" - to be chief operations officer and get original theater productions going.

Wolfensohn began his career as a lawyer and investment banker in his birthplace, Sydney, Australia, moved on to a London investment bank and finally became a partner of Salomon Brothers in New York. He became a U.S. citizen in 1980.

Among his most spectacular successes was the financial bailout of Chrysler Corp. in the seventies.

He became a board member of Carnegie Hall in New York in 1973 and chairman of the board in 1980. As Carnegie chairman he was approached by the Kennedy Center for advice on picking a chairman and eventually persuaded to take the job himself.

Even in Washington, noted for its driven, workaholic administrators and legislators, Wolfensohn is seen as something of a phenomenon.

He is constantly on the move, visiting a country or two a day for his investment firm, then popping back to Washington to put in two days a week at the Kennedy.

In between his responsibilities as a financier and impresario he has an astonishing array of hobbies. He fences and was a member of the Australian Olympic fencing team in 1956. He fishes, paints, sails and plays tennis.

He took up cello playing in his early 40s and progressed fast enough to mark his fiftieth birthday with a public performance on the Carnegie stage accompanied by Isaac Stern on violin and Daniel Barenboim and Vladimir Ashkenazy on piano.

His family worries about this punishing pace but sees little sign of his easing up. His daughter Sara, a concert pianist, says, "I particularly worry when he gets a grey-skinned look."

But Wolfensohn makes clear he is not considering dropping anything right now and above all not his Kennedy Center tasks. "It's much more fun than investment banking," he says.