The half-million federal-postal workers who will be shifted into a new pension program next year will get an unexpected bonus from the government: a starter contribution -- equal to 1 percent of their salary plus interest -- to a new tax-deferred investment program account.

That gift from the government for workers hired since January 1984 could range between a few hundred and a few thousand dollars, depending on the employe's length of service and salary.

For example, a worker who joined the government in January 1984 with an average salary of $20,000 would have $600, plus interest, placed in his or her starter investment account next year.

President Reagan signed the new pension plan into law two weeks ago.

It goes into effect in January. Benefits for workers under it will come from Social Security, a modified civil service pension and earnings from the optional tax-deferred investment program, which is being offered to government workers for the first time.

Federal workers who were hired before 1983 may continue under the current civil service retirement system (which does not include Social Security or offer an investment plan), or elect to come into the new system.

They will not, however, get the "starter" account bonus from Uncle Sam.

A little-noticed provision of the new retirement act requires the government next year to contribute starter money to each mandatorily covered employe's thrift account equal to 1 percent of salary from the first day of employment on or after January 1984.

The investment plan -- which is similar to, but more generous than, most 401(k) plans offered to some workers in the private sector -- will permit employes to put up to 10 percent of their salaries each year into an investment account. The government will put in a contribution of up to 6 percent based on the employe's contribution.

Money in those accounts will be tax-deferred until retirement or until the employe leaves government, when it could be rolled over into an IRA.

Employes may put lesser amounts into the investment plan.

Those who contribute nothing will still have 1 percent of salary put in for them by the government. Early Retirement

Still no movement on the Senate bill to offer an early-retirement window to federal and postal employes this year.

The Senate and the White House are negotiating terms of the bill. The House plans no action until a bill is okayed by the Senate.

The original plan, by Sens. William V. Roth Jr. (R-Del.) and Ted Stevens (R-Alaska), would have waived normal age and service requirements between July 1 and Dec. 31. Employes could retire on immediate annuity at any age after 25 years' service; at age 50 after 20 years' service; at age 55 after 15 years' service, or at 57 after five years' service. Employes would take a 2 percent pension cut for each year they were under age 55 at retirement, and agencies would be barred from hiring replacements for most employes.

Federal and postal unions object to the hiring freeze in the bill. The administration would like more flexibility in hiring and would like to give agencies control over which employes could take advantage of an early-out. Because of the objections -- and lack of House interest -- there is little chance the early-out will be offered this year.