Plans to convert the former Robert Bergstrom Air Force Base in Austin, Texas, into a commercial airport are expected to provide a boost for the city's high- tech shippers.

The cargo terminal at the existing airfield, Robert Mueller Municipal Airport, is already operating at full capacity, often forcing carriers to divert much of their Austin freight to Dallas, Houston and San Antonio. Cargo volume at the facility has been growing 20 percent to 30 percent since 1992."The current facilities are very small and very inadequate," said Doug McCasland, traffic and transportation manager for Motorola, which employs 10,000 people at its two facilities in Austin.

"Hopefully the new airport will allow large volumes of freight to come directly into Austin as opposed to Dallas or Houston," he added.

Besides Motorola, other high-tech manufacturers with operations in and around the Texas capital include Dell Computers, Texas Instruments, 3M Corp. and CompuAdd Corp.

"It's vital that we get new capacity," said Bruce Byron, director of government relations for the Austin Chamber of Commerce.

Ground-breaking for a new cargo terminal at Austin-Bergstrom International Airport is slated for Jan. 1, with all-cargo carriers expected to begin operations at the facility in the fall of 1996.

"I think it will be a good move," said Gary Milton, district manager for Burlington Air Express in Austin, adding, however, "we want to make sure that our costs don't go up."

Burlington's business in Austin has kept growing despite the limitations at Robert Mueller, but it sometimes has to divert freight to Dallas or Houston, he said.

Passenger airlines will continue to use the cargo facilities at the old airport until the new one opens for passenger traffic in late 1998.

Gen Weest, traffic manager for CompuAdd Corp., a manufacturer of computers and cash registers, said delays in picking up freight at the old airport

because of its space constraints have sometimes set back production at CompuAdd's facilities by four hours.

"Austin's a big international city," she said, noting that some 50 percent of its air cargo is either import or export traffic.

Alan Graham, a local real estate developer who is one of the two partners in CargoPort, the name of the new terminal, said it will initially have at least 200,000 square feet of storage space, but that it will have room to expand to 400,000 square feet.

The cargo ramp will have parking space for up to 20 aircraft, including 747s.

"The project we're building is not a 'build it and they will come' type," Mr. Graham said. "It's predicated on existing demand, both in and out. Our manufacturing base is such that companies like

Federal Express, Burlington and Airborne have demand here."

Under an agreement with the Federal Aviation Administration, the old airport will be closed for aviation once the new airport opens for passenger operations, Mr. Byron said. No decision has yet been reached on how it will be used, but the Chamber of Commerce official noted that it's close to both the University of Texas and downtown Austin and would be a good location for multiple uses, including residential, recreational and industrial.

Ray Brimble, Mr. Graham's partner in CargoPort, said the terminal's design will apply modern storage and distribution concepts, enabling it to serve as ''a major economic engine well into the 21st century."

The new facility is expected to cost $23 million. Mr. Graham said it will be financed through the sale of tax-exempt revenue bonds.

Bergstrom closed as an active air force base in September 1993, but some reserve activities have continued there. The Defense Base Closure and Realignment Commission voted in June to shut down reserve operations as well.