More than 230,000 government workers in half a dozen agencies are due retroactive pay averaging $20 because payroll offices failed to adopt a bookkeeping change that took effect last October.

Agencies that shortchanged employes include the Department of Health and Human Services, State Department, Federal Communications Commission, Defense Logistics Agency, Defense Contract Audit Agency and the General Services Administration.

Thus far, only GSA has corrected its payroll, according to Office of Personnel Management officials.

The strange back pay problem has its origins in the Omnibus Budget Reconciliation Act of 1982. Congress ordered federal agencies to begin computing the salaries of white-collar civil servants on the basis of a 2,087-hour work year.

Previously, agencies had figured employe pay rates on 2,080 hours of work annually.

Congress made the payroll change effective for about two years and said it was to expire Sept. 30, 1985, unless Congress extended it again.

The rationale was that the bookkeeping change would save the government millions of dollars, have a minimum impact on employes and more correctly reflect pay rates.

When the change went into effect in January 1984, it cut the hourly pay rate for most workers. The highest-paid employes lost about $8.80 every two weeks, while the average civil servant's hourly rate cut worked out to about $3.20 per pay period.

However, because a 3 1/2 percent pay raise went into effect at the same time, many federal workers failed to notice that their hourly rates had dropped.

Last September, just days before the 2,087-hour system was to expire, OPM alerted all federal agencies that Congress was unlikely to extend the system and told them to prepare to return to the 2,080-hour system for computing salaries.

But several agencies failed to get the word.

Last month Rep. Barbara Mikulski (D-Md.) asked OPM to see how many agencies had failed to revert to the old pay computation formula. She acted on complaints from Ed O'Connor, an official of the American Federation of Government Employees local at the Social Security Administration in Baltimore.

Just how, or when, employes will be reimbursed hasn't been determined. All in the Family?

A Washington-based association says that some of its members are getting the association's monthly magazine, The Gazette, so late that a lot of its news and information is stale. That sort of thing happens, but it is especially embarrassing for this particular group.

"Since we know The Gazette is being mailed on a timely schedule, it is apparent that the problem is occuring somewhere in the delivery system, and we are attempting to find out where," says the National Association of Postmasters.

Many of the customers who are getting their magazines late are the people who run the nation's midsized and big post offices. People

Ed Evans has taken over as communications director for the National Federation of Federal Employees. Evans, a longtime South Carolina television journalist with good Capitol Hill connections, had been with the Federal Managers Association.