It was believed a chemical from painting at the nearby hospital tower block was circulated through the air conditioning.
The APEX union, which represents allied, scientific and technical employees in New Zealand, said at least 30 to 40 staff members were affected.
Advocate Omar Hamed said workers suffered headaches, dizziness, coughing, tightness of airways and skin conditions such as rashes and sensitivity to chemicals.
"One of our members was admitted to ICU after a particularly bad exposure event in the hospital," he said.
The union believed the fumes leak happened between February and September this year.
Hamed said hospital management's response was inadequate during the first half of the year.
"We understand the reports that our members were inputting into the risk management incident reporting system were essentially not being responded to.
"In the last two to three weeks Te Whatu Ora's national health and safety service and the new local deputy chief executive have become involved, they stood up an incident management team, and it appears now they have now a really comprehensive response in place both supporting staff to get the medical advice that they need and navigate ACC claims."
Hamed said this situation highlighted the importance of Te Whatu Ora having a national, easily accessible reporting system.
Health New Zealand Te Waipounamu deputy chief executive Martin Keogh said he was "deeply concerned" by the reports of staff becoming ill.
"The fumes are believed to be Hexamethylene Di-Isocyanate (HDI), which is being used in painting work nearby," he said.
"However, all construction activity on the Christchurch Hospital Campus is currently paused until we have established the exact cause. It will not start again until I am sure that we are providing a safe environment for everyone."
Keogh said they had not received any reports of any potential exposure for patients, but one staff member had been hospitalised and others reported a range of symptoms including breathing problems, rashes and headaches.
"I understand the severity of the situation and am deeply concerned about the impact it has had on our staff. All staff have access to appropriate support - this includes the appointment of a specialist Doctor who is undertaking clinical assessments of those affected," he said.
"We believe the issue could have started earlier this year, but as part of a full investigation we are in the process of establishing exact timelines."
WorkSafe had been notified.
