The New York City Conflicts of Interest Board is fining the city planning commissioner, Dolly Williams, $4,000 for failing to recuse herself from decisions regarding the giant Atlantic Yards development near downtown Brooklyn, the board announced yesterday.

Despite her critical role in approving the project, Ms. Williams in 2004 invested $250,000 in the New Jersey Nets, which is owned by the developer of Atlantic Yards, Forest City Ratner Companies. The Nets are scheduled to move into a new arena as part of the $4 billion development, which will also add thousands of units of housing and office and retail space to the low-rise neighborhood.

Ms. Williams, the CFO of A. Williams Construction, signed a statement with the Conflicts of Interest Board admitting that she had invested in the Nets just weeks before she voted in favor of the downtown Brooklyn redevelopment plan, which would benefit some of the land to be included in the Atlantic Yards project. The decision by the board comes more than three years after the Brooklyn Paper originally reported that Ms. Williams’s financial holdings posed a conflict of interest. After her relationship with the developer was disclosed, Ms. Williams recused herself from future involvement with the project, including design recommendations in 2006 that gave the commission’s stamp of approval to the plan.

The president of Brooklyn, Marty Markowitz, announced last month he would replace Ms. Williams. Yesterday, he introduced her replacement: Shirley McRae, the chairwoman of Brooklyn’s Community Board 2, which includes the Atlantic Yards development.

Yesterday, the state named Forrest Taylor, who served as chief of staff to a former City Council speaker, Gifford Miller, to fill a newly created position as ombudsman for the Atlantic Yards development.

Daniel Goldstein, a spokesman for a coalition group opposing the Atlantic Yards project, Develop Don’t Destroy Brooklyn, said he welcomed both the change on the planning commission and Mr. Taylor’s new role as ombudsman.

“We hope Mr. Taylor is eager to get started right away because tons of questions from the community have been piling up over the past year, and they need to be answered,” Mr. Goldstein said. He cited security concerns and details on the project’s boundaries and timeline as areas that he felt had not been adequately explained by the developer.

As for the new commissioner: “It’s got to be an improvement over someone who’s just been fined over conflicts of interest,” Mr. Goldstein said. He added that he was encouraged by Ms. McRae’s critical perspective on the Atlantic Yards issue during her time on the local community board.