The Senate Intelligence Committee has requested internal Treasury Department documents concerning CIA Director William J. Casey's work for Indonesia in 1976 in an effort to determine whether he should have registered as a foreign agent.

In a letter to Treasury Secretary Donald T. Regan, Committee Chairman Barry Goldwater (R-Ariz.) said the inquiries center on reports that Casey and other members of his New York law firm had met with Treasury officials "to discuss tax credits for oil exploration and other matters related to the government of Indonesia."

The request was made as part of the committee's unfinished investigation into Casey's financial activities during the last decade. The investigation was launched in mid-July following the resignation of Casey's handpicked chief of covert operations, Max Hugel, for alleged improper stock market activities.

Casey's firm, Rogers & Wells, registered in July, 1977, as an agent for the Indonesian government. In a statement to Justice Department's Foreign Agents Registration Office, the firm said it was undertaking "legal representation before U.S. governmental departments . . . in connection with obtaining U.S. foreign tax credit for Indonesian income taxes paid by U.S. oil companies."

At the time, U.S. oil companies could not write off taxes on Indonesian oil extraction as they could in Middle Eastern countries, a knowledgeable official said yesterday. The Internal Revenue Service had specifically disallowed foreign tax credits under production-sharing contracts used by the Indonesian government at the time.

Casey and his firm were hired to bring about a change in that ruling, but unlike the firm, Casey never registered as a foreign agent.

Casey failed to list the Indonesian government or many of his other legal clients in connection with Senate confirmation proceedings early this year despite an intelligence committee rule calling for such disclosures over the last five years.

Instead, he provided a copy of a short list of clients that had been submitted to the Office of Government Ethics under less rigorous disclosure rules. Casey submitted a fuller list Aug. 27 at the Senate committee's request.

CIA General Counsel Stanley Sporkin said yesterday that he believed Casey had contacted Treasury and IRS officials on behalf of Indonesia but had done "no lobbying" and had not participated in other activities that would have required him to register.

Moreover, Sporkin contended that Casey's activities at the time were not covered by the Foreign Agents Registration Act because of an exemption enacted in 1966 for lawyers engaged "in the legal representation of a disclosed foreign principal before any court of law or any agency of the government of the United States."

According to Sporkin, Rogers & Wells registered "later on"--in 1977--only because "they anticipated they might be going beyond "the attorney's exemption" to take part in "a legislative approach" to the problem. By then, he said, Casey was no longer working on the case.

Attempts to reach senior members of Rogers & Wells to determine how much time and work Casey billed to the Indonesian account and in what years were unsuccessful. In his supplemental disclosure Aug. 27, Casey simply listed Indonesia and many others as clients for whom he had "billable time or otherwise received credit" during the period 1976-81.