Federal and postal employes are putting an average of $5 million per day of their own money into the new tax-deferred thrift investment program.

As of close of business yesterday the fund -- which is invested in long-term guaranteed Treasury securities -- had $349 million.

Money in the fund includes $120 million invested by workers as of June 15, and more than $4 million in earnings. The remainder represents government contributions to the fund, including retroactive payments to the accounts of most workers who have joined the government since the start of 1984. Most of those employes are automatically covered by the new Federal Employees Retirement System.

Under the thrift investment plan, which started in April, civil servants covered by the new Federal Employees Retirement System can put up to 10 percent of salary, or a maximum of $7,000 this year, into the tax-deferred program. Those investing at least 5 percent get a matching 5 percent tax-deferred contribution to their accounts from their agency. Workers under the FERS system get an automatic 1 percent government contribution to their accounts every pay day whether they invest or not.

Employes covered by the older Civil Service Retirement System (the approximately 2 million federal workers hired before 1984) can put in up to 5 percent of this year's salary into the thrift plan. They are not eligible for matching contributions from the government, however, unless they switch to the FERS pension plan. The open season for making that switch begins Wednesday and will run through Dec. 31.

All money invested this year goes into a special G-Fund made up of long-term (four years or more) Treasury offerings. The interest rate changes monthly. In April the fund was paying 7 5/8 percent. In May the interest rate went to 8 3/8 percent and this month it is 8 5/8 percent.

Starting next year workers covered by the FERS plan can split their investments among these three options: the G-fund, a guaranteed investment fund paying a flat rate of interest, and a stock market fund with the rate of return pegged to the market's performance.

Like private sector thrift plans, the federal program is subject to an annual antidiscrimination test by the Internal Revenue Service. Rules limit the contribution rate of upper income (those making $50,000 per year or more) to no more than 2 percent above the average contribution rate of all eligible lower income workers. Depending on total participation that test could raise -- or lower -- the amount upper- income federal workers can invest.

So far about 28.9 percent, or 162,943 of the 562,970 workers covered by the FERS plan, have invested their own money in the thrift plan. The Federal Thrift Investment Retirement Board, which is managing the plan, said that 296,826 of the approximately 2 million workers covered by the old pension plan had also enrolled in the tax-deferred investment program as of mid-June.Pension Planning

Starting next month U.S. workers covered by the old CSRS pension plan will have the option of changing to the new FERS system.

Federal agencies have promised to give free pension planning help to all workers. But many employes are seeking help from private firms that, for a fee, give them computer printouts showing their benefits under both systems. At 1 p.m. tomorrow on WNTR radio (1050 AM), representatives of SORT Inc. will give free computer benefit readings to callers.