The incident occurred around 10 a.m. Friday at 435 U.S. Route 130.
Several people reported feeling dizzy and nauseated, and one person passed out.
12 people were being treated at the scene - they did not have to go to the hospital.
The workers described a strong, sweet odor at the time.
Investigators have conducted air quality tests, but say so far they have not found anything. The New Jersey State Police hazmat unit has been called in to bring additional - and more sensitive - equipment.
There is no word on what may have caused this situation. Investigators say no spills or powder residue have been found.
There was a heightened sense of concern, considering this facility was the scene of an anthrax attack back in 2001.
Meanwhile, officials say residents have been calling, concerned their mail has been contaminated. Officials say mail coming out of the facility has been screened for biohazards, so they are confident there is no danger in opening mail.
Suspected HazMat situation at US post office Rte 130 Hamilton. Men in moonsuits Inspecting mail truck pic.twitter.com/lRe6a0iTMN— Nora Muchanic (@Muchanic6abc) May 15, 2015