The Carter administration is on the verge of allowing construction of the controversial Seabrook, N.H., nuclear power plant, which has become a nationwide symbol in the battle over atomic energy.

Environmental Protection Agency Administration Douglas M. Costle acknowledged that he "made a final decision" to allow the plant to discharge 1.2 billion gallons of heated water into the Atlantic Ocean daily - an issue which has held up construction since November.

However, he added in an interview that announcement of the decision has been delayed several days because "I want to go through the record one more time to make absolutely sure."

The $2 billion Seabrook plant - while it was actually halted on grounds of water pollution rather than nuclear danger - has been considered a test of President Carter's nuclear policy.

Anti-nuclear activists who have fought the plant in the courts, in regulatory hearings and in demonstrations for five years have pointed to Carter's campaign promise to make nuclear energy "a last resort."

Pro-nuclear forces, including New Hampshire Gov. Meldrim Thompson Jr. and Public Service Co. of New Hampshire, which is building the plant, say it will provide crucial electricity to New England and relieve dependence on Arab oil - another Carter goal.

The decision to go ahead with the plant conforms with the administration's emerging nuclear policy, as outlined in the energy program: to discourage the use of plutonium, from which bombs can be made, but to promote the building of conventional reactors such as Seabrook.

Another situation where a nuclear plant conflicted with environmental goals was also recently decided in favor of the plant: Interior Secretary Cecil D. Andrus decided not to fight a proposed Indiana reactor on the edge of Indiana Dunes National Lakeshore.

The 2,300 megawatt Seabrook plant had been granted a license by the Nuclear Regulatory Commission and was under construction last November when EPA's regional administrator in Boston ruled there had to be modifications in cooling water system to protect the delicate ecology Seabrook's estuary.

The plant would discharge a daily volume of water equal to the flow of a large river, 39 degrees hotter than ocean temperature. Large numbers of fish and aquatic organisms would be drawn into the 19-foot wide water tunels and "friend," McGlennon contended after reading through 5,000 pages of administrative hearing testimony by the utility and environmental witnesses.

The utility appealed McGlennon's decision to EPA Administrator Russel Train, who left it to the Carter administration to decide.

"It's a very complicated case that has to be decided on very narrow legal grounds," Costle said, adding that he has "painstakingly" read through the administrative hearing record.

"It has to do with the sufficiency of the record," he said. "The central issue is, can I find from that record the effect one way or another" of the plant on the environment.

Costle's decision was scheduled to be announced early Friday afternoon. But the announcement was abruptly postponed until later Friday and then until this week, after consultation with the White House, EPA officials said.

Costle denied that the decision was postponed because the President did not want to answer questions about it at his scheduled news conference today. He said he was busy on Capitol Hill and with White House appointments and "I ran out of time."

The Seabrook question was bound to be controversial whichever way it was decided. Last month, 1,414 demonstrators were arrested when they tried to occupy the plant site. They oppose the plant because it would release low-level radiation, because it is being built near an ancient earthquake fault and in their view might cause a major radiation accident, and because the government has not yet developed a program to store radioactive waste.

Nuclear proponents, on the other hand, say the government is strangling energy production in red tape. When McGlennon stopped the plant, 500 construction workers had to be laid off and the dalay has cost the utility $15 million a month.