A lawyer for the D.C. government's gasoline supplier said yesterday that the city is losing thousands of dollars in "prompt payment discounts" because it fails to pay its gasoline bills within 30 days.

"The District has never paid promptly," said Bryan Perilman, an attorney for Roarda Inc., a Maryland firm with a contract to supply the city with 5.2 million gallons of gasoline this year.

The city is now about $750,000 behind in payments to Roarda and sometimes takes six months to pay bills, according to Perilman -- a situation that could cost the city an extra $78,000 under the terms of its contract.

City officials confirmed these figures yesterday, saying that about $250,000 is more than 30 days overdue to Roarda.

If the city pays Roarda within 30 days, it qualifies for a 2 percent discount on the gasoline it buys.

At a City Council committee hearing Wednesday, it was disclosed that the District pays Roarda 3 to 20 cents more for a gallon of gasoline than the price found at local gas stations -- minus taxes. (The District pays no tax to Roarda on the gas.)

In addition, the District pays 18 cents a gallon more than Fairfax County and 11 cents more than Montgomery County.

If the city paid on time, its price would be more than 2 cents a gallon less.

As city officials and the company tried yesterday to sort out the reasons for Roarda's high price to the city, a top city contracting officer said the District will ask the U.S. Department of Energy to audit Roarda's prices "to assure the District gets a fair shake."

Under federal price regulations and its contract with the District, Roarda may raise its prices only as much as its supplier, Gulf, raises its prices to the firm.

It appears that Roarda has done that and nothing more, according to a check yesterday of Gulf price increases against Roarda's price increases to the city.

The Roarda contract was awarded Aug. 1, 1979, at these prices: 83 cents a gallon for regular, 88 cents a gallon for premium, and 87 cents for unleaded. aBy Feb. 25, Roarda had increased those prices to $1.10 1/2 for regular, $1.15 1/2 for premium and $1.13 1/2 for unleaded.

The increases were, respectively, 27.5 cents, 27.5 cents and 26.5 cents -- precisely the increases that Gulf put into effect nationwide during that same period, according to a Gulf spokesman in Houston who was asked about Gulf prices without being told anything about Roarda and the District. a

The Aug. 1 contract with Roarda was negotiated without any competitive bidding because, a city procurement memorandum said, "Gasoline is not available from any other source during the period of (national gasoline shortage)."

"We are married to Roarda," said city procurement officer Kenneth E. Quinlan, adding that under federal gasoline allocation regulations, Roarda is required to supply gasoline to the city but no other supplier is required to do so.

Both Quinlan and Perilman, the Roarda attorney, said yesterday that the city could, in fact, go to other suppliers for gasoline. But Quinlan said their price might be even higher in "periods of crunch," and that their supply might dry up altogether since it would not be guaranteed.

City officials and Perilman said yesterday that the prices that the city negotiated with Roarda on Aug. 1, 1979, took into account the difficulties of making gasoline deliveries to District facilities.

While gasoline is typically delivered in 7,000-gallon "tankwagon" loads to service stations, Roarda must use smaller trucks to make deliveries efficiently to the 84 tanks specified in the contract.

These tanks range in size from 300 to 10,000 gallons or more and are located in the "nooks and crannies" of the District, Quinlan said, with sites ranging from Lorton prison in Virginia to Glenn Dale Sanitarium in Maryland. Perilman said this kind of delivery adds pennies per gallon to the cost of the gasoline.

In contract, Fairfax and Montgomery counties receive their gasoline in the big tankwagon loads, and Fairfax even sends its own trucks to pick up the gasoline.