The first South African to refuse to be drafted into his country's armed forces on the grounds that they are fighting unjust wars was sentenced to one year's imprisonment by a court-martial here today.

William G. Paddock, a 31-year-old former candidate for the priesthood in the Anglican Church, told the court: "I cannot enter the Defense Force because of the role it plays in defending the structural violence of the South African system."

It was half the sentence Paddock might have received under a defense law that precludes any form of conscientious objection.

Associates of Paddock and other observers sympathetic to him were surprised by the relative leniency of the sentence. They speculated that it might reflect a desire by military chiefs not to make a martyr of Paddock or highlight his case and a concern that draft dodging could snowball among political opponents of the government's strict segregationist policies, called apartheid.

The Defense Force's chief of personnel, Lt. Gen. R. F. Holtzhausen, expressed some of this concern when he told a society of student counselors Sept. 30 there was a growing movement against the draft on most South African university campuses.

"There is increasing pressure on our young men to dodge national service or to question the underlying motive," Holtzhausen said.

Under South Africa's conscription system, every white youth on leaving school or university has to do an initial two years in the Defense Force, followed by several days of reserve duty each year until he is 60.

After the first five years on reserve, when he is liable for call-up to active service in an "operational area" like Namibia, the citizen-soldier has to do 12 days a year in a local "commando," similiar to a national guard unit, until he is 60. It adds up to nearly 4 1/2 years' soldiering if he starts when he is 17.

A small but growing number have left the country to avoid the draft, some seeking political asylum abroad, mostly in Europe.

Paddock is the first to make a stand of public defiance on overtly political grounds.

He looked conspicuously unmilitary when he appeared at the court martial today, with a bushy beard and a casual corduroy jacket among the heavily braided Army officers. The president of the court martial was Col. Elias L. de Villiers.

Paddock conducted his own case, refusing the help of an Army lawyer, and called Bishop Desmond Tutu, the black general secretary of the South African Council of Churches, as his only supporting witness.

In a lengthy statement, Paddock cited eight criteria for a just war drawn up by St. Augustine and argued that the South African Defense Force failed all of them.

He said the war being fought in Namibia, also known as South-West Africa, against guerrillas of the South-West Africa People's Organization (SWAPO) is unjust because the South African Army is an illegal occupying force in a foreign country.

South Africa's continued presence in Namibia has been declared illegal by the International Court of Justice, he said, and the court's opinion had been endorsed by the U.N. Security Council. Paddock argued that the South African government lacked legitimacy at home too because the African population, 70 percent of the total, has no vote and is excluded from consultation on the country's constitutional structure.