The Treasury Department goofed a week ago and sent out 400 checks with overpayments totaling $5.368 million to buyers of U.S. securities, officials said yesterday.

The good news, however, is that 69 of the investors who received the windfall from Washington already have returned nearly $1 million that they received by mistake.

Treasury officials said they hoped to recover all of the overpayment within three weeks. "We feel these people are basically honest," explained a red-faced bureaucrat.

If the recipients of the extra-large checks fail to coopeate, the Treasury will have the last word. It simply will cancel the Treasury bills, and the purchasers will be left empty-handed when the securities mature a year from now.

A computer programming error was blamed for the foul-up when advance interest payments were mailed Nov. 14 to buyers of the securities.

As a result, the buyer of a $10,000 Treasury bill received a check for $8,819 instead of the correct interest payment of $1,177.70. That amounted to an overpayment of $7,641.30 for the typical purchaser.

The error was detected after 400 checks were mailed. Persons on the overpaid list were telephoned immediately, and follow-up letters were written asking the recipients to return their check or send a personal check to repay the windfall.

"So far, we have got back $970,726, and some people have returned the check even without getting our notice," said delighted Richard W. Gregg, director of the division of financing of the Bureau of Public Debt.