Three more people have been rushed to hospital from an Auckland office building after being struck by a mystery illness, less than 12 hours after an unexplained suspected gas leak hospitalised a dozen workers from the same building earlier today.

The Augusta Building on Victoria St West in Central Auckland was closed to the public tonight as emergency services were forced to evacuate the building following a second callout.

It was unknown when the building will be habitable again.

Emergency services received initial reports that two patients were convulsing - however this was later debunked by Fire and Emergency NZ staff.

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The whole building was being systematically evacuated around 7pm, and will remain off limits until further notice.

A St John Ambulance spokeswoman confirmed another three people were taken from Augusta Building, on Victoria St West, to Auckland hospital, after falling ill.

Eight more were assessed at the scene by St John staff.

St John was called to the scene shortly after 5.30pm. Three ambulances and three medical managers were at the scene.

Fire and Emergency NZ's Hazmat team were also in attendance.

Meanwhile, an investigation is underway to get to the bottom of what caused the illnesses.

The Augusta Building on Victoria Street West in Central Auckland was closed last night after emergency services were forced to evacuate the building following a second callout. Photo / NZME
The Augusta Building on Victoria Street West in Central Auckland was closed last night after emergency services were forced to evacuate the building following a second callout. Photo / NZME

In the earlier incident, patients were rushed to Auckland Hospital complaining of nausea and shortness of breath after four floors of the building were evacuated around 9am.

Some of the patients were also vomiting following the incident.

Fire and Emergency New Zealand confirmed at least two fire engines returned to the scene just before 6pm.

They could not confirm whether anyone had been in the building, but said firefighters were inside.

A spokesman from Augusta Building said the landlord and building manager would be assisting Auckland Regional Public Health Service and Worksafe 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, and the building had been cleared for re-entry before the second callout.

Fire and Emergency's assistant area commander Dave Woon said building owners were getting an independent company in to check the air conditioning.

That was yet to be organised.

"Our measuring devices have not been able to pick anything up, explosive ordinance disposal have not picked anything up. There is little point in us going through that process again."

Gas and chemical detectors came up with nothing, he said, and the cause of the drama was still a mystery.

"That's not to say the building owners won't be looking at it, and they may find something that we've missed, but I'd be surprised if they did."

Despite that, Woon said it was clear something in the building had caused the sickness, due to the similarity in the symptoms.

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

A spokesman for the company said Qantas would be investigating - but that currently 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."