In another ominous sign for the Carter administration's anti-inflation efforts, the Labor Department reported yesterday that the rate of increase in union wages is rising for the first time since 1975.

The increase came despite recent moderating of wage increase rates for the work force as a whole and the administration's highly visible efforts to focus on big union contract settlements in trying to win compliance with its voluntary wage restraint program.

Administration anti-inflation officials have seen the leveling-off of overall wage rate increases as a cause for optimism. But several economists, both inside and outside the government, suggested yesterday that the new upsurge in union wage settlements could trigger "catch-up" pressures in the nonunion work force as well.

The Labor Department report found that first-year wage increases in contracts negotiated during the first six months of 1979 averaged 8.2 percent, up from 7.6 percent during 1978. Wage increases negotiated for each year over the life of new contracts showed a comparable rise - 7 percent, as opposed to 6.4 percent last year.

Increases in combined wage and benefit adjustments amounted to 9.2 percent for the first year and 7.3 percent annually during the contract, compared to percentages of 8.3 and 6.3, respectively, for last year.

None of the figures includes projected cost-of-living increases, which, in the 57 percent of contracts that have cost-of-living clauses, are expected to add substantially to wages because of the current double-digit inflation rate.

For this and other reasons, the figures, compiled by the Labor Department's Bureau of Labor tstatistics, are not suitable for comparison with the administration's guideline of 7 percent a year for wage and benefit increases. But if cost-of-living payments are included, as they are in the guideline formula, the wage averages appear to be considerably in excess of 7 percent.

The averages for the first six months of this year reverse a four-year moderating trend in the rate of increases in union wages. Not since 1975, when the size of first-year wage increases began dropping from a peak of 10.3 percent, has there been an increase in the wage-rise rate.

In an ironic twist, the biggest upward pressure on wages came in the one area where anti-inflation officials had expected little trouble: construction. Nonunion competition and unemployment had kept union construction wages in check since the early 1970s and were expected to continue to do so.

But, instead, construction union wage increases averaged 9.2 percent for the first year, up from 6.5 percent last year. When benefits are included, the increase was even greater.

Manufacturing wage gains also were up sharply - 9.6 percent as opposed to 8.3 percent last year. But the growth of construction union wages was so disproportionately strong that when wages for all industries except construction were averaged, virtually no change in the rate of increase was found.

tthe Labor Department report included several large recent union settlements, including the Teamster master freight pact, but not even more costly settlements covering the rubber industry and some airlines, which will show up in subsequent reports.

The report covered nearly 1.4 million workers, about one-third of them from the transportation industry, including trucking, and one-quarter more from construction.

The Bureau of Labor Statistics counts only contracts cov ering 1,000 or more workers in computing wage averages and 5,000 or more workers in computing combined wage-benefit averages.