Rep. John Conyers Jr. (D-Mich.) said yesterday he had fresh evidence that South Africa tested a nuclear bomb in the South Atlantic Ocean almost six years ago, but the Tennessee doctor whose material Conyers was using challenged the assumption it was the telltale sign of an atomic test.

Presenting what he described as declassified federal documents obtained under the Freedom of Information Act, Conyers said they showed that radioactive fallout rose sharply in Australia almost two months after the suspected South African nuclear test near the island of Marion 1,500 miles south of the Cape of Good Hope.

Conyers said the evidence showed that Australian sheep had ingested unusual levels of radioactive iodine, which he said was the first time animals in Australia had suffered the fallout effects of a nuclear test.

"I'm not saying the new evidence is the smoking gun in this case," Conyers said at a news conference, "but I do think it's enough to reopen the issue."

The evidence offered by Conyers was a report by the antiapartheid Washington Office on Africa, which cited thyroid studies on slaughtered Australian sheep by Dr. L. Van Middlesworth of the College of Medicine at the University of Tennessee. The report said the Washington Office on Africa found the evidence in 500 pages of declassified documents from the Naval Research Laboratory, whose director had corresponded with Middlesworth.

"The report says this was the first time I detected radioactive iodine in the thyroids of Australian sheep, which is not the case," Middlesworth said in a telephone interview. "I routinely find it in sheep after every French atmospheric test in the South Pacific."

The doctor, who said he has been examining the thyroids of sheep for signs of radioactivity since 1954 under contract from what is now the Department of Energy, said he had found that radioactive iodine in the thyroids of Australian sheep had risen to six times their normal level in November 1979, less than two months after a U.S. Air Force satellite named Vega had witnessed what appeared to be an atomic explosion in the South Atlantic above Antarctica.

"Six times the normal level of radioactivity is not a sharp increase," Middlesworth said. "We've seen increases of 1,000 to 10,000 times after the French tests, which were sharp increases."

Still, Middlesworth said, he is puzzled by the findings. "We studied those thyroids in detail because I could not get away from the fact that there had been an increase just after the time the South African test was suspected. But I never published my findings. I have never been able to decide whether that rise in radioactive thyroids was real or not. I was never convinced they were the results of fallout from an atomic test."

A White House panel convened by then-President Jimmy Carter came to the same conclusion, deciding after months of deliberation that the findings presented to it by various government agencies were insufficient evidence of a nuclear test.