The destruction of key White House documents on the Iran-contra affair began in early October -- more than a month earlier than previously disclosed -- after Lt. Col. Oliver L. North and then-CIA Director William J. Casey concluded "this whole thing was coming unraveled," the former National Security Council aide testified yesterday.

Earlier testimony described shredding by North and his secretary Fawn Hall on Nov. 21, but North told the congressional Iran-contra committees that he and Casey decided that "things ought to be cleaned up" after two unexpected events took place early last October.

On Oct. 5, a C123K cargo plane was shot down over Nicaragua, leading to the capture of American crewman Eugene Hasenfus and the disclosure of an air operation to ferry arms to the Nicaraguan contras. Later it was revealed that North had secretly organized and directed the operation.

Two days after the cargo plane incident, Roy M. Furmark, a New York businessman and former law client of Casey, told the CIA director that funds from the secret arms sales to Iran had been diverted to the contras and that two businessmen who had helped finance some of the deals were threatening a lawsuit that would expose the operation, according to an account Casey gave Congress last December.

During six hours of testimony yesterday, North provided new details about administration attempts to cover up the secret arms deals with Iran and the subsequent diversion of profits for the benefit of the contras. North's testimony also provided new evidence that Casey and former national security advisers Robert C. McFarlane and John M. Poindexter attemped to obscure the role of President Reagan and other officials in the Iran arms sales.

North's references to frequent discussions with Casey, including private one-on-one encounters, seemed to back up a theory advanced by some congressional investigators -- and supported by McFarlane -- that the late CIA director and North had a close working relationship that operated outside normal channels.

However, some congressional panel members have criticized the North-Casey theory as too convenient in light of the fact that Casey is unable to present his version.

Previous testimony had indicated that cover-up attempts by administration officials had begun in mid-November, after a pro-Syrian weekly in Lebanon disclosed on Nov. 3 that McFarlane had led a secret delegation to Tehran. The report touched off a flurry of news stories in the United States.

Witnesses have testified that administration officials began assembling a false chronology of the Iran arms deals during the week of last Nov. 17. The altered chronologies attempted to hide the fact that Reagan and other administration officials approved Israel's shipment of U.S. weapons to Iran in the late summer and fall of 1985.

Congressional investigators have attempted to show that administration officials were especially concerned about these shipments because they were carried out in apparent violation of the Arms Export Control Act, which governs transfer of U.S. arms to third countries.

Until yesterday, testimony about the shredding, particularly from McFarlane and Hall, had focused on Nov. 21, the same day North learned that Attorney General Edwin Meese III had begun an inquiry into the Iran arms sales.

But North said yesterday that by Nov. 21 he already had shredded a sizable amount of material. "We saw these operations unraveling as early as the mid part of October with the loss of the Hasenfus airplane and the discussion that the director of central intelligence had had with a private citizen about what he knew of a contra diversion," North said. "I began to . . . recognize I would {be} leaving the NSC, because {there} was a purpose for my departure, to offer the scapegoat, if you will."

The former White House aide said that although he had shredded documents "almost every day" while at the NSC, he "started cleaning things up . . . in earnest" in mid-October.

He said that among documents he set out to destroy were about five memos he sent to Poindexter seeking presidential approval for arms deals that included references to diversion of funds to aid the contras. North said that while he assumed Reagan had approved his proposals, he was never told nor did he receive written verification that the president had given authorization.

North said he was "quite sure" that the five memos were destroyed before the Meese inquiry began.

North acknowledged under questioning that the Nov. 21 shredding proceeded "perhaps with increased intensity" after he learned that Meese's aides were going to review NSC files the next day.

North testified that on Nov. 21 he "had assured Adm. Poindexter -- incorrectly, it turns out -- that all of the documents that pertained to the residual funds being used to support the Nicaraguan resistance had already been destroyed."

But on that same day, according to earlier testimony by McFarlane, North told McFarlane he was going to organize a "shredding party."

On Nov. 22, aides to Meese discovered a memo in North's files outlining the planned diversion. When Meese interviewed North the next day, North expressed surprise that the memo was "still around."

"I thought I had gotten it all," North said.