WorkSafe is investigating the mystery fumes in a central Auckland office building that put 15 people in hospital yesterday with symptoms such as headaches, vomiting and nausea.
Another 100 people were assessed on site but numerous tests failed to establish what the problem was at Augusta House in Victoria Street West.
Worksafe said its investigation could take some time.
Vector also ran comprehensive tests this morning but didn't find any traces of natural gas.
Tenants have been barred from the building until the cause of fumes have been identified.
Augusta House on the corner of Victoria Street West and Elliot Street was evacuated twice yesterday - once in the morning and again in the evening.
Office workers complained of an unusual smell but numerous tests failed to establish what the problem was.
Ambulance officers assessed 109 more people at the scene.
First reports were of people suffering nausea, headaches and vomiting after an unusual smell on the eighth floor of Augusta House.
Ambulance crews were called back about 5.30pm after more people had similar symptoms.
City Hospital said it treated 15 patients with mild to moderate symptoms in its emergency department yesterday.
It said they were observed for four to eight hours before being sent home.
Qantas said yesterday that nine of the patients were their workers.
Last night, Fire and Emergency assistant area commander, Dave Woon, said the cause was still unknown.
Mr Woon said he was satisfied with the decision to let people back in the building, even though more fell ill from mystery fumes.
The owners of the building have bought in independent experts to check the air conditioning.
The owners said they won't let anyone back into the building until they have a definitive answer about what caused the leak.
John Crocker from Unite Union, which represents Qantas staff, said he was absolutely shocked when he heard what happened.
"We sent one of our field officers out there immediately to monitor the situation," he said.
"There was a lot of confusion, we're still not entirely sure what happened."
Mr Crocker said there was increasing frustration among Qantas workers who were not being allowed back inside.
"They're hanging around in the city, not really knowing what's going on, when they're going back to work or when they should call it a day and head home," he said.
"They're a little bit afraid, they don't want injury.
"They're obviously frustrated by this ... while this investigation goes on, they're in the dark [like] everyone else."
It was a serious situation, Mr Crocker said.
"The most important thing is the safety of our workers, we want to be completely satisfied that everything's safe before they re-enter their workplace," he said.
"We've go to make sure that nothing like that happens again."
The Auckland Regional Public Health Service said it was following up with people who had been unwell.
In a statement, its medical officer of health Dr Denise Barnfather said staff were interviewing those worst affected to get a clearer picture about where they were in the building when they were exposed to the chemical and what symptoms they experienced.
No one should enter the restricted areas of the building until they were told it was safe, she said.
"People who were exposed to the chemical and are experiencing symptoms such as burning in the eyes or back of the throat, shortness of breath or nausea and vomiting should seek medical attention immediately," the statement said.