Council's customer centre has re-opened.
The office area where the mail was opened remains cordoned off for now and a temporary call centre has been set up at the library.
Call centre staff will work from there until it is known if the substance found was hazardous.
Police will notify Council when the results of an initial test come through.
Chief executive Geoff Williams said he was very pleased with staff's response to the incident.
"Based on information from the Fire Service and police we're confident the substance won't turn out to have been hazardous - but we have to treat the situation as though it were," he said.
"Staff who were directly affected dealt with the situation very well but it was traumatic for them.
"Specialist cleaners have decontaminated the mail area and immediate vicinity and we'll keep air conditioning off in that part of the building until we know more about the substance. If it's found to be hazardous we will get air conditioning specialists to clean the system.
"Other than that, it's business as usual today."