The Fire Hazchem team isolated the envelope and its contents, then decontaminated the scene, she said.
She said the item was handed to police in a secure biohazard container and has now been sent for forensic examination.
Emergency services were alerted to the incident around 8.30am.
A cordon was in place between Cameron Rd and Third Ave, which was closed for several hours as police and hazardous materials experts removed the suspicious package from the building.
Tauranga mayor Greg Brownless said the incident was "abhorrent" and "sickening."
"The point isn't whether [the powder] turns out to be dangerous or not, the intent is the same - to intimidate."
Brownless said he would not condone threats to the council.
"Nowadays it seems people can't discuss the things they disagree about without going off the wall."
Both Brownless and Susan Jamieson, the council's newly-appointed general manager of people and engagement, praised the quick actions of the mailroom staff who found the envelope and followed protocols.
Jamieson said the council was treating the incident seriously, as instructed by the police, however, it was not assuming the intent behind the package.
She said all staff members involved in the incident were in good health and being appropriately supported.
Other staff were being kept informed when news was available. She said the police had told the council to expect it may be up to a week before the substance was identified.
Council staff could feel confident they were safe at work, she said. The council was not putting any additional security measures in place.
As was standard after an incident, the council was reviewing its protocols for suspicious packages.
