Although the Spoleto Festival U.S.A. is primarily devoted to music and dance, this year it presented the American premieres of two new Australian plays, performed by the original companies. The Sydney Theater Company offered ''The Perfectionist,'' an investigation of modern marriage by David Williamson. The Handspan Theater of Melbourne made its debut with a sound-and-light show called ''Secrets.'' Despite the high-tech electronic effects of ''Secrets,'' it was the traditional play ''The Perfectionist'' that proved to be the more venturesome dramatic work.

One of Australia's foremost contemporary playwrights, Mr. Williamson is known in America as a screenwriter of large-scale films (''The Year of Living Dangerously,'' ''Gallipoli''). As he demonstrates in ''The Perfectionist,'' he is also an astute, economical observer of those universal subjects, husbands and wives, parents and children, in shared discord.

Behind the Moli ere-like title is a group portrait of the mutual destructiveness that can exist within families: how fathers can set impossibly high standards for offspring to emulate, how a son-as-husband can repress a wife and children while pursuing his own self-interested goals.

The perfectionist of the title refers doubly to father and son, the father an autocratic Australian barrister, the son an academic who is otherwise engaged with an endless thesis that shatters the stability of his marriage. The heroine of the evening is the son's wife, a woman with a rising consciousness and sense of fair play.

The author is, if anything, too fairminded. He tries to present all sides of every issue, including negative fallout from rising feminism and house-husbandry. At times, the evening is more schematic than dramatic, as the characters become illustrative of antithetical positions. But the dialogue has the drollness of a Simon Gray, and the play is the opposite of provincial. It is a definite advancement over ''Players,'' the last Williamson play to be seen in America.

In ''The Perfectionist,'' he is asking for an independence of mind, body and bond. Each of the characters refuses to be categorized by his peers. The son, for example, begins by parroting his father's male supremacy with such statements as, ''My achievements are for all of us,'' but gradually realizes that gratification can come if one ''renegotiates'' for marital equity.

The staging by Rodney Fisher frames the play in an uncluttered environment, using minimal modern furniture, with characterizations acting as atmosphere. As the barrister and his wife, Noel Ferrier and Diana Davidson give a full picture of a kind of natural Australian aristocracy of social position. Peter Carroll and Robyn Nevin are closely wedded as the younger couple, and Hugo Weaving is equally convincing in the catalytic role of a handsome young Dane, a Marxist of elastic propriety. The five offer the evening close-quarter authenticity.

While Mr. Williamson and his actors are tantalizing us with domestic truths, Nigel Triffitt and the Handspan Theater are reprocessing visual tricks from the experimental theater. This non-narrative multimedia puppet show has a few arresting images - a baby carriage pushed by disembodied hands; a climactic, laser- beam starburst that impales the audience in its gaze.

However, the connective material is gossamer-thin and the techniques should be familiar to those who have followed the work of the Bread and Puppet Theater and Ralph Lee, or have marched in the Greenwich Village Halloween parade. As director and designer, Mr. Triffitt has heightened his spook show with a melange of found sounds and musical strains. All this is submerged in a cloud of stage fog, a smokescreen substitute for theatrical inventiveness.

As the fog creeps over ''Secrets,'' it is a relief to move from artistic pretension to Piccolo Spoleto, a busy fringe festival of theatrical and musical events. Piccolo performers include clowns, mimes and magicians. In a diversion entitled ''Phantasy, Phun & Phoolishness,'' two young American clowns, Carl Asch and Phil Joseph, warm up an audience with an hour of spontaneous frivolity. They are country kin to the Flying Karamazov Brothers, and should that juggling band wish to expand its family, these two should be the first phools to apply.

At Spoleto U.S.A. THE PERFECTIONIST, by David Williamson; directed by Rodney Fisher; set designer, Shaun Gurton; lighting designer, John Rayment; stage manager, Nick Schleiper; technical supervisor, Paul Staples; coordinator, Ann Churchill-Brown. The Sydney Theater Company at the Spoleto Festival U.S.A., Charleston, S. C. Barbara GunnRobyn Nevin Stuart GunnPeter Carroll Erik LarsenHugo Weaving Jack GunnNoel Ferrier Shirley GunnDiana Davidson and SECRETS, conceived, designed and directed by Nigel Triffitt; realization, Kenneth Evans; executive producer, Helen Rickards; technical manager, Phillip Lethlean. The Handspan Theater. WITH: Winston Appleyard, Andrew Hansen, Peter Wilson, Lizz Talbot and John Rogers.