About 150 workers were moved from Australia Post's international mail centre at Granville, seven people were assessed by paramedics, and two were treated after the spill on Wednesday afternoon, Fire and Rescue NSW Superintendent Ian Krimmer said.
Twenty firefighters are at the scene.
"We're just in the process of using sand to soak up the spillage," Superintendent Krimmer said.
Authorities are "yet to fully identify the nature of the substance", he added.
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