Union members had asked their SkyCity bosses to see the report from health officials, who authorised it was safe for business to resume.
Unite senior organiser Joe Carolan said it was "absolutely shocking" SkyCity was encouraging workers to come back to work when its corporate team wasn't going back until Tuesday, and other companies in the area weren't letting anyone back until later this week.
Barnett, who suffers from hay fever and respiratory problems, said she had not seen anything in writing and felt uncomfortable relying on word of mouth.
"We need a detailed explanation about what was in the fumes and the steps taken to test the safety of it."
She said the long-term effects of the toxicity were as much of a concern, if not more, as the immediate dangers.
"Back in the day, my father worked in fumes of asbestos. Years later, his lungs are collapsing. My concern is not understanding what is in the air we are breathing.
"I think the intelligence of the workers is being insulted."
Though Barnett had not come back to work since the fire broke out on Tuesday, she had been into SkyCity to help a friend move her car on Thursday.
"You could smell the toxicity in the air. I don't understand how it is safe.
"I was due in last night but after 99 per cent of us voted in favour of the strike action I stayed home."
Barnett was due back into work tonight but was undecided if she would go.
"That would depend on what action we get from our bosses."
Yesterday, a SkyCity spokesperson said: "We have completed a really thorough health and safety check. We have followed out a number of very thorough checks.
"We've had an independent third party come in, a hygienist, and signed it all off and authorised it. We've worked with WorkSafe.
"Of course they have a right to strike but we are 100 per cent confident that our business is safe and welcome for customers to come in."
The spokesperson said emergency officials had confirmed on Thursday the fire was under control and it was safe for business to resume.
"Air quality has been rigorously monitored over the past three days, consistently meeting all regulatory standards, at no stage showing any level of threat."
The Herald has approached WorkSafe for comment.