The Census Bureau yesterday won an important round in its long legal battle to release final 1980 census figures by Dec. 31, but another court fight remains.

Supreme Court Justice Potter Stewart ordered a delay in a lower court ruling that the Census Bureau must adjust figures for Detroit to compensate for an undercount before releasing final figures, but a related ruling in New York was not affected by Stewart's order.

Census officials were elated by Stewart's ruling and hoped it might affect the New York case. "All it means to us so far is that the Detroit case is no barrier to reporting the final count," said Henry Smith, a census spokesman. "But in everyday terms, we're not a lot closer today than we were."

"The government feels that it cannot turn over the figures until there is a stay in the New York ruling, but we are trying to get one," said Kenneth S. Geller, deputy solicitor general.

A total of 29 suits have been filed against the Census Bureau, most charging that the government undercounted blacks, Hispanics and other minorities. Because seats in Congress, and millions of dollars of federal aid are apportioned on the basis of cenus figures, cities and states that have lost population stand to lose votes in Congress and considerable money if the preliminary census figures become official.

Census Bureau Director Vincent P. Barabba has disputed charges that the 1980 census seriously undercounted population in the nation's big cities, calling the 1980 count "by far the most complete . . . in our history."

Preliminary national census figures released earlier this month showed a population of 226 million and a larger-than-expected shift in population from the North and Midwest to the South and West.

Even if the figures are reported by Dec. 31 -- the date the Census Bureau says it is required by law to submit them to the president -- they could be changed later, depending on the outcome of the pending suits.

"If the plaintiffs prevail in these cases, there is not a shred of doubt that we will abide by those ruling," Geller said.

But Census Bureau officials have argued in court that they need to report the final 1980 figures by Dec. 31 to allow state legislatures to begin the process of reapportionment. Preliminary estimates by the bureau indicated that 10 states in the South and West will gain 14 congressional seats, although the figure could go as high as 17.

"A lot of state legislatures need those figures," Geller said yesterday.

In its brief before the Supreme Court, the government listed 12 states that is said needed the figures by early 1981, including Virginia, which joined the government in its request for a stay of the Detroit ruling. In addition, Texas, which stands to gain at least tow seats because of an estimated 26 percent increase in population, has announced it may have to put off reapportionment until after the 1982 elections unless the census figures are released in time for its biennial legislative session this spring.

Detroit sued the Census Bureau the day after the counting began last spring. On Sept. 25, U.S. District Court Judge Horace W. Gilmore barred the Census Bureau from reporting 1980 population figures until statistical adjustments were made for persons not counted.

On Tuesday, U.S. District Court Judge Henry Werker said the Census Bureau would have to readjust its figures to compensate for what he called a "disproportionate undercount" in New York City and State. Earlier he had ordered the government not to release the final figures as scheduled on Dec. 31. Werker's ruling was affirmed by the Court of Appeals. The government has since asked the Court of Appeals to rehear the case and issue a stay on the part of the preliminary order that would prevent release of the figures on Dec. 31.

Geller said yesterday the government had asked the Court of Appeals to take note of Stewart's ruling in the Detroit case.

If the government were to lose there, it likely would appeal to the Supeme Court.