Liverpool City Council, in Sydney's southwest, says five people working from its Moore St office have developed thyroid cancer across the last three to five years.
A sixth person has reported a thyroid condition but has not consented to take part in a NSW Health department investigation.
Three of the cases were found on level six of the office, with that floor shut down and staff relocated to level four while the probe takes place.
The council has engaged a third-party environmental health consultant while the Health department runs a separate epidemiological investigation.
"The decision to relocate staff is in response to the council's high level of concern for the welfare of its staff," the council said in a statement.
"Council takes this very seriously and future action will be based on the results of the two concurrent investigations."
The United Services Union called for more action including clearing the Moore St building completely.
"We don't know what we are dealing with here, all workers in the Moore St building need to be rehoused immediately, I can't put it any more plainly," USU acting general secretary Daniel Papps said.
"We are very concerned that a number of our members are sick and their welfare and that of their colleagues is the only thing that matters right now."
Jenny Havilah, one of the workers who is still recovering from thyroid cancer surgery, admitted she was concerned for everyone in the office.
"It certainly sounds very sinister … I'm worried about my colleagues, not just on the sixth floor (but) that worked in that building," she told Nine News.
"I had my thyroid and some lymph nodes removed and I'll get the results in a week or so."
The council told Nine it had been informed it was "unlikely there is a causal connection between our workplace and the cancer diagnosis".
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