Justice Clarence Thomas failed to publicly disclose additional private travel provided by the wealthy conservative donor Harlan Crow, a top Democratic senator said in a letter on Monday.

Customs and Border Protection records revealed that the justice and his wife, Virginia Thomas, took a round trip between Hawaii and New Zealand in November 2010 on Mr. Crow’s private jet, according to the letter. Senator Ron Wyden, Democrat of Oregon, writing to Mr. Crow’s lawyer, demanded that he supply more information about the financial relationship between the two men.

The letter, part of an inquiry that Mr. Wyden, the chairman of the Senate Finance Committee, has opened into Mr. Crow and the justice, comes as top Democrats have urged major changes to the Supreme Court, including an enforceable code of conduct.

Mr. Wyden said the latest revelation had only increased his misgivings about the relationship between the justice and Mr. Crow, a real estate magnate. “I am deeply concerned that Mr. Crow may have been showering a public official with extravagant gifts, then writing off those gifts to lower his tax bill,” Mr. Wyden wrote.

Justice Thomas did not immediately respond to a request for comment on Monday.

A spokesman for Mr. Crow, Michael Zona, said that his lawyers had “already addressed Senator Wyden’s inquiries, which have no legal basis and are only intended to harass a private citizen.” It accused Mr. Wyden of a politically motivated effort to undermine the Supreme Court.