The number of Christchurch Hospital staff potentially exposed to toxic fumes from a construction site has increased to 55.
Health union APEX believed an industrial chemical from painting at the nearby hospital tower block was circulated through the air conditioning.
Workers in the radiology and ultrasound departments suffered headaches, breathing problems and rashes, and one had to be admitted to intensive care.
Health NZ Te Waipounamu deputy chief executive Martin Keogh said the leak was believed to have started earlier this year, and 55 staff had come forward so far.
The agency had engaged an external independent investigator to carry out a full investigation.
"As part of this investigation, the exact timelines of when the issue started and how it has been managed to date will be established. We believe the issue started earlier this year and additional controls for the air conditioning system were in place following internal investigations, however further reports of symptoms have led us to take these next steps to confirm the root cause and ensure we are providing a safe environment for everyone," Keogh said.
Construction was paused on the Christchurch Hospital campus in the meantime.
APEX, which represents allied, scientific and technical employees in New Zealand, believed the fumes leak started in February.
Earlier this week advocate Omar Hamed told RNZ at least 30 to 40 staff members had become unwell.
He said hospital management's response was inadequate during the first half of this 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."
RNZ obtained a memo sent to staff on Tuesday by Keogh, that stated the fumes may have originated from spray-painting of fire-retardant paint products being applied to the steel structure of the Tower 3 site.
"Because the root cause of the fumes experienced by radiology staff remains unclear at this time, I have ordered all construction works across the entire Christchurch Hospital campus that creates and generates fumes to stop until we have clarity all works are safe to resume.
"A number of radiology staff have developed symptoms which include a range of respiratory and/or skin/rash symptoms, ranging in severity from very minor to very serious, including one staff member who was admitted to the Intensive Care Unit."
The memo said Te Whatu Ora was working hard to identify staff or patients who may have been exposed.
It talked about hexamethylene diisocyanate (HDI), what the substance actually was and what the health risks were.
The memo said it was used as a thickener and hardener for two-pot paints. It could cause irritation of the respiratory tract, dizziness, irritation and headaches.
There could be long-term immunological reactions, which could cause adult-onset asthma, skin reactions and gastro.
It also said for the pregnant staff, there is no risk that HDI is teratogenic (causes changes in unborn children) or carcinogenic (able to cause cancer).
