Government employes would be offered a private sector-style pension package with generous tax breaks under proposed legislation being drafted by the Office of Personnel Management.

Under the plan, which must still clear the Office of Management and Budget, all federal and postal workers who are fully covered by Social Security (anyone hired since January 1984) would automatically go under the new system. The earliest civil service pension benefits would come at age 59 1/2, and reduced benefits would be available from Social Security at age 62, or full benefits at age 65 -- the same requirements persons in the private sector have.

Employes covered by the current civil service retirement program -- which unlike the OPM plan guarantees a level of benefits based on length of service and salary -- could remain with it or elect to buy into the new system.

The package must clear a number of political hurdles -- from federal unions to Congress -- and be approved by the Internal Revenue Service, which could be expected to take a dim view of some of its tax-deferred features.

Under the proposal:

* All contributions to the civil service retirement program would be made by the government. Employes would pay nothing. Currently, workers contribute 7 percent of salary.

* Employes would keep the same salary they have now, but the government would contribute an amount equal to 11 percent of each employe's salary into his or her individual retirement account. Interest would be paid on those accounts at the Treasury bill rate, which runs about 10 percent. If OPM gets its way, that money would not be considered as income and subject to federal taxes until the employe retired or withdrew it.

* All employes in the new system would pay the full Social Security tax.

* If it can run the IRS gantlet, the OPM would also offer a special tax-deferred savings plan. Workers could contribute up to $5,000 into that program. That is more than twice the tax saving available to persons under individual retirement accounts.

* Workers would be vested in the new retirement plan after one year. Money in their accounts could not be withdrawn until they were at least age 59 1/2. If they left the government, money in their accounts would continue to draw interest (but with no new government contributions) until they could withdraw it. Employes would get annual statements of the value of their accounts.

* There would be no such thing as a formal retirement age under the new program, although benefits could not begin until age 59 1/2. Currently, federal workers can retire as early as age 55 with 30 years' service, but in fact federal employes on the average work until about age 61.

OPM officials say the new plan would cost the government the equivalent of 19 percent of payroll, compared with the current plan, which costs 33 percent of payroll.

"It the current system is very generous only for the very few who make a full career of government," an OPM official said yesterday.