An investigation into allegations of personal misconduct forced postponement yesterday of the confirmation hearing for President Carter's nominee to become the first black federal judge in the Deep South.

Sen. Dennis DeConcini (D-Ariz.) said he called off the hearing after the committee received reports of serious charges against Robert F. Collins, a 46-year-old magistrate from New Orleans. "The little I've seen is hearsay and from informants, but because of the nature of the charges I felt the committee had to conduct a thorough investigation in conjunction with the FBI," DeConcini said.

Francis W. Rosenberger, the committee's chief of staff, said yesterday that he was dispatching two of his top investigators to New Orleans to check out the charges, which involes allegations about Collins' private life and his conduct on the bench.

Collins, one of the first blck graduates of Louisiana State University law school and an active civil rights attorney for years, could not be reached for comment yesterday. But Rosenberger said he talked to Collins for two hours yesterday and he categorically denied the charges.

Michael J. Egan asscociate attorney general who handles judgeship nominations, said that the allegations against Collins were received by the FBI last week, shortly after he was nominated by Carter.

Collins already had been through a routine FBI backgorund check, but the case was reopened, egan said, "so all that will be behind when he-'s confirmed."

Egan said that no thought has been given to withdrawing Collins' nomination.