About 600 federal employes received a total of $7 million in overtime payments - the equivalent of about six months' worth of overtime each - during a recent one-year period, congressional investigators said yesterday.

The investigators, lacing their testimony with details of fictitious employes and charges of payroll abuse or possible fraud, told a congressional subcommittee that the federal government's overtime pay controls are poorly managed andchaotic.

Among the details disclosed at a hearing of the House compensation and employe benefits subcommittee were these:

A supposedly safeguarded computer at the Department of Housing and Urban Development recently issued a $99,000 paycheck to a fictitious employe named Donald Duck in a trick test by investigators.

One worker at the Department of Health, Education and Welfare whose base salary is $13,158 collected $18,441 in additional pay for overtime.

In one instance at the National Institutes of Health, when only seven people were needed for weekend overtime duty in a laundry room, managers assigned 21 people to work overtime for what were described as "morale reasons."

At the Defense Department, in 50 percent of cases involving overtime pay, such overtime work was "not properly justified" because it wasn't authorized or forms were incorrectly filled out, internal auditors found.

These and other revelations yesterday raised serious questions about the state of the government's payroll security procedures, the potential for fraud and abuse, and managers' policies on using overtime instead of hiring parttime workers or turning to other alternatives, the congressional investigators said.

During fiscal 1977, the government - excluding the Postal Service - spent about $1 billion for overtime, and yet no one has previously taken a good look at the way this is handled, according to Rep. Gladys Spellman (D-Md.), chairman of the subcommittee.

Overtime figures compiled by federal agencies at the request of the subcommittee "scared the hell" out of agency officials, an aide to Spellman said. "Now they have all started investigations of their own."

Several cases of overtime pay fraud or abuse at the U.S. Forest Service, the U.S. Civil Rights Commission, the Internal Revenue Service, the Department of Transportation, and other agencies already are in various stages of investigation or prosecution by federal officials.

But the problem goes beyond a few isolated, possibly criminal cases to the system itself, Spellman said.

The revelations come at a time when fraud and waste by government workers is a political issue. The General Accounting Office (GAO), the congressional watchdog agency, recently warned that agencies have not been doing enough to defect fraud, and that the Justice Department had been slow to help them.

Under a 28-year-old law, all federal accounting systems are required to be approved by GAO, and yet 48 percent of them remain unapproved, according to testimony yesterday by H. L. Krieger, a GAO investigator. Others have allowed their systems to deteriorate over the years, he said.

The two "most derelict" agencies - Defense and HEW - dispurse almost 50 percent of the U.S. government budget, through their "unapproved accounting systems," said one GAO witness, John J. Cronin.

In what was referred to as "the Walt Disney affair," GAO officials said they had been able to trick payroll computers at HUD into making "clearly unreasonable payments to fictitious individuals" such as Mr. Duck, Mickey and Minnie Mouse and, in all, 30 cartoon characters. The computer should have rejected a $99,000 check to Duck on its face, since the maximum federal pay is $47,500, and because it is supposed to have other procedural safeguards, Cronin said.

"An employe skilled in the use of a computer could do the same thing using a fictitious name," he said.

Officials from HEW, the Department of Agriculture and its Forest Service, the IRS, and the Commission on Civil Rights also testified yesterday. All vowed to try to improve their accounting systems.

Officials of the Justice and Defense departments will testify at a hearing on Nov. 3.

Of the 600 employes who received collectively $7 million in overtime payments, 458 worked for the Justice Department.

The subcommittee will not turn any cases of possible overtime fraud over to the Justice Department for prosecution "until we find out what their explanation is" for the huge amounts of overtime paid to these Justice Department employes, higher than at any of seven agencies surveyed employes, so far, a Spellman aide said.

"It's a question of who polices the policemen," Spellman said.

No one at the Justice Department who was familiar with the overtime situation could be reached for comment. But a subcommittee aide said the 458 employes came from all levels of the department, from GS5 to GS14, and from various subdivisions, but that none works for the FBI.

Limits on the numbers of federal employes recently further tightened by President Carter), loss of human control to automated computers, a lack of support to agencies from the Office of Management and Budget, and frequent changes in top management at federal agencies were among the factors cited by GAO officials as contributing to the plight of government accounting systems and the use of overtime pay.