Airdrome Orchards Inc., an orange grower and packer group based in San Jose, Calif., is used to sending most of its navel-orange exports to Japan.

But this year, for the first time in Airdrome's history, more of its navels are headed for Australia than Japan."Somebody tried our fruit and liked it," said Chuck Fumia, Airdrome's secretary-treasurer and part-owner of the group's 1,000-acre citrus farm.

Airdrome, which sells its product under the Big Jim label, is one of several navel-orange growers benefiting from the boost in exports to Australia.

Sunkist Growers Inc., California's largest marketing cooperative for citrus fruit growers, estimates that its navel-orange exports to Australia and New Zealand will climb 15 percent in 1990 to about 600,000 40-pound cartons

from about 500,000 cartons last year.

Australians have become enamored of the navel orange because it is sweeter and juicier than locally grown varieties.

Florida produces more navel oranges than any other state, but its production has declined in recent years. California, meanwhile, has been increasing its production and is now at two-thirds the level of Florida.

U.S. exporters of navel oranges are looking beyond Australia to other parts of the Pacific.

U.S. exports to Japan, the Pacific Rim's largest importer of U.S. citrus, are much higher than those to Australia, but the Japanese market is relatively flat. Sunkist said its navel-orange exports to Japan will likely remain at about 11 million cartons in 1990, roughly the same as last year's level.

Spurring exports from the West Coast will be a bumper crop expected in California. This year's crop also has larger fruit, which Asian consumers favor. Texas and Florida output, meanwhile, is expected to drop because of freezes in late 1989.

Frank Limacher, agricultural economist with the California Department of Food and Agriculture, said that as Japan allows more citrus imports into the country by 1991 in response to trade pressure, the United States could lose out.

In effect, the opening of the Japanese market could pave the way for other overseas competitors to gain market share.

In 1989, the United States exported 11.9 million pounds of oranges to Australia, up from 11.5 million pounds a year earlier. These totals are not tracked by individual variety, so statistics are not available on total navel exports.

California's bumper year, meanwhile, could have its problems. Harvesting the fruit is expected to take longer because of the huge volume.

The harvest usually is completed by May, but this year it could last into July. This would expose the fruit to the elements, thus impairing quality, said William Martinet, an economist at Sunkist.

In addition, the time spent harvesting the crop late in the season could detract from the care and management of other crops.