The Press reported the fumes came from an industrial chemical circulated through the air conditioning.
The source of the fumes is believed to have been a chemical spray used on the new hospital tower currently under construction.
The Association of Professional and Executive Employees union warned about the "ongoing chemical fumes" in a newsletter sent to its members.
"A chemical flame retardant and volatile organic chemical, which was being sprayed on the building, was drifting off the building site, and being pulled into the hospital through the air conditioning vent," the newsletter said.
"The fumes were circulated straight into the hospital, with the closest services to the vent - radiology and ultrasound - bearing the brunt of the poisonous fumes.
"Over the course of some months, multiple staff members have been affected by symptoms congruent with toxic fume exposure, including at least one needing ICU admission."
Te Whatu Ora temporarily halted work on the new hospital campus while it investigates the cause of the fumes and ensures the area is safe, The Press reported.
Construction work on the six-storey tower started in April last year and it is due to be completed by mid-2025.
