NEW YORK - Talks to avert a New Year's Day strike by thousands of office-building janitors, doormen and elevator operators have resumed.

Members of Local 32BJ of the Service Employees International Union and the Realty Advisory Board returned to the bargaining table yesterday. Both sides say they are prepared to talk through the weekend, if necessary.

The workers, who average $40,500 a year in wages and $55,000 a year when health and pension benefits are included, say that the industry has been enjoying boom times but that they have not benefited from them.

Industry officials say the city already has the highest building- worker salaries in the nation.

If the workers walk out, about 2,000 buildings would be affected, including the World Financial Center in lower Manhattan and the MetLife building in midtown. *