An investigation is under way after 15 people were hospitalised from exposure to an unknown chemical in an Auckland office building.

People were rushed to Auckland City Hospital complaining of nausea and shortness of breath after four floors of the Augusta Building on Victoria St West in central Auckland were evacuated about 9am yesterday due to a suspected gas leak.

Three more people were taken to hospital with mystery illnesses about 5.30pm after a second suspected gas leak.

An Auckland City Hospital spokeswoman this morning said 15 people were brought in yesterday with mild to moderate symptoms, and were treated in the emergency department for between four and eight hours.

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The patients had all been discharged.

A spokeswoman for Auckland Regional Public Health Service said they were interviewing people "worst affected" after being exposed to an unknown chemical yesterday.

"The interviews are to determine where they were at the time of exposure and what symptoms they experienced," she said.

ARPHS Medical Officer of Health Dr Denise Barnfather recommended nobody entered the restricted areas of the building until they were deemed safe.

"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," she said.

WorkSafe New Zealand is investigating the incident.

Fire and Emergency assistant area commander Dave Woon said last night firefighters along with the Defence Force used specialised equipment to try to detect any hazardous substances, but they could not find anything.

This morning they had handed the incident over to the property owner and had notified WorkSafe and the Auckland Regional Public Health Service.

The building was placed in lockdown last night, but now the situation had been handed over to the property owner, Woon said.

"[It is] for them to make a call as to whether it is reopened or not."

The Fire Hazmat unit outside Augusta House on Hobson St, central Auckland, after another gas leak was reported. Photo / Ryan Dunlop
The Fire Hazmat unit outside Augusta House on Hobson St, central Auckland, after another gas leak was reported. Photo / Ryan Dunlop

This morning the foyer of Augusta Building appeared open and people were going up and down the building using the elevators.

A spokesman for the building said the landlord had undertaken further testing overnight to ensure the premise was is "operable for business as usual" today.

They would be carrying out additional "non-evasive testing" today.

The landlord and building manager would be assisting with an investigation into the incident.

Preliminary testing carried out by FENZ, police and the New Zealand Defence Force had not identified the source of the smell, he said.

"Subsequent oxygen tests are normal with zero pollutants identified in the contamination testing."

Tenants initially evacuated the building as a precaution after the first incident, and the building had been cleared for re-entry before the second callout.

Qantas said several people at its office in the building had become unwell after fumes became apparent about 9am.

A spokesman for the company said Qantas would be investigating - but that the focus was on the welfare of the affected employees.

"Emergency services attended the scene and a number of people were taken to hospital in a stable condition," he said.

"They are our first concern and we are monitoring their progress."