Less than two months into the new fiscal year, the Defense Department is facing a deficit of at least $325 million in overseas allowances for servicemen because of the tumbling dollar.

Defense Department Comptroller Robert W. Helm said the Pentagon had been caught by surprise -- just as businessmen had -- by the dollar's rapid fall the past month. The dollar's value against such currencies as the Japanese yen and West German mark is already roughly 20 percent below the Pentagon estimate at the start of fiscal 1988, he said.

Those declines are forcing the Pentagon to increase every month the overseas housing and cost-of-living allowances of military personnel, Helm said.

The allowances vary by rank, family size and foreign country but are essential "to give them a comparable living standard wherever they are overseas with what they might have here," Helm said.

Even if the dollar does not sink any lower, the allowances have risen to significant levels. An Army major assigned to Tokyo with three dependents, for example, is now receiving a monthly cost-of-living adjustment of $1,182 in addition to his basic pay. An Army sergeant with three dependents is drawing $738.

"Taking a snapshot right now, we're looking at $325 million in unbudgeted costs in the military personnel accounts. If the dollar does something else, that's going to change. But based on the current drop, we're looking at $325 million right now."

The Pentagon has devised a plan to cover that growing shortage but needs congressional approval to transfer funds among various accounts, Helm said. Without that flexibility, the Defense Department will be forced to submit a supplemental budget request to Congress this fiscal year, despite the drive to reduce the federal deficit, he said.

The Pentagon has little choice because it is required by law to pay overseas housing allowances and foreign cost-of-living adjustments to military personnel stationed outside the United States, he said.

Helm said the department had begun fiscal 1988 on Oct. 1 with a budget of $777.4 million for these overseas allowances. Roughly 500,000 of the 2.1 million men and women on active duty are deployed outside the United States.

According to the comptroller, the Pentagon's budget assumed the dollar would buy 2.06 German marks and 163.10 yen. Due to the financial turmoil that began with last month's stock market dive, the dollar is now buying only 1.65 marks and about 134 yen -- declines of 20 percent and 18 percent, respectively.

Unless the dollar falls even more sharply, Helm said, the allowance deficit could be covered by tapping another account known as the currency fluctuation fund. That fund was established by Congress in 1979 to provide extra money when needed to pay for higher operating costs at overseas military bases.

The currency fluctuation fund receives its money from so-called "expiring balances" -- money appropriated by Congress in previous years for weapons buying and operations and maintenance that turned out not to be needed.

"What we're asking Congress now is to allow us to use balances in the fluctuation fund for overseas station allowances . . . so that we don't have to go up to the Hill and ask for a supplemental {appropriation}," Helm said.