Firefighters called back to an Auckland high-rise following a suspected gas leak earlier.

Twelve more people have been affected by a mystery gas leak at an Auckland high-rise, eight hours after it hospitalised 12 people on Thursday morning.

Fire and Emergency NZ spokesman Scott Osmond said firefighters had been called to Augusta House in Auckland's CBD just before 6pm.

Three fire trucks and a hazardous materials unit were sent to the scene.

STUFF Fire trucks were back outside the Augusta Building in central Auckland on Thursday evening after more people became ill.

The incident, on level 7, seemed to be a "repeat" of the morning's events, which saw 12 people sent to hospital with nausea, shortness of breath and vomiting, Osmond said.

Firefighters and the bomb squad gave the building the all-clear after that incident.

"Hopefully someone will get to the bottom of this," Osmond said.

LAINE MOGER/STUFF Three ambulances were at the scene.

Three people were taken to hospital, and nine people were assessed at the scene on Thursday evening, Fire and Emergency assistant area commander Dave Woon said.

St John Ambulance area commander Simon Barnett said all the patients were stable and were not seriously injured.

The building was being evacuated and would be closed for the night.

Initial reports that people were suffering convulsions were not correct, and they in fact had "very similar" symptoms to those displayed on Thursday morning, Woon said.

Firefighters would not attempt to identify or measure any hazardous gases or chemicals at the building overnight.

"We tried today, we had the best tools that we had today. In the cold light of day we'll have another look at it," he said.

"I believe we've covered all our bases previously . . . we've missed something.

"There's obviously something wrong, and we've got to get to the bottom of it."

St John Ambulance spokeswoman Melissa Peters said paramedics received a call at 5.37pm.

The patients taken to hospital were in a moderate condition, she said.

Three ambulances were at the scene.

One lane of Victoria St West had been blocked, and police were diverting traffic.

A Qantas spokesperson said two staff members at the building had reported feeling unwell and had been assessed by ambulance officers.

The office was now closed, the spokesperson said.

"We're continuing to work with the property manager and authorities to find out the cause of the original smell."

Woon said he did not know how many people had been in the building when the patients reported feeling unwell.

A REPEAT OF THE MORNING

STUFF Emergency services give media stand up after toxic leak in central Auckland

Earlier, the bomb squad gave the building the all-clear after the mystery gas hospitalised 12 people.

Fire and ambulance crews were called to Augusta House about 9.35am for what was thought to be a suspected gas leak.

Exposure to the unknown substance caused 12 people to come down with nausea, vomiting and shortness of breath, but exactly what it was remains a mystery.

ADAM JACOBSON/STUFF The suspected gas leak injured 12 people in Auckland CBD.

Woon said fire crews ran gas detectors throughout the building and did not come up with anything.

The New Zealand Defence Force's Explosive Ordnance Disposal (EOD) – the bomb squad – was then called in to test the scene.

"We couldn't find anything that triggered our devices . . . and a preliminary search by them hasn't found anything either," Woon said.

CHRIS SKELTON/STUFF Paramedics had set up a triage area to assess the health of people working in the Victoria St West building.

The smell had gone, making it difficult for Fire and Emergency to figure out what he happened, he said.

Woon said they were going to turn the air-conditioning back on again to test the system.

"Our best detectors aren't detecting anything so I couldn't guess how this has happened – I will say that the symptoms [vomiting] that are being shown by a couple of people have been quite significant so it's not something that we're taking lightly."

CHRIS SKELTON/STUFF Entry to the Augusta Building was being blocked by cones.

Five people were initially taken to hospital but that number had risen to 12 by 2pm, as more people were assessed.

About 9am, several employees at Qantas Airways office became unwell, a Qantas spokesperson said.

"At this stage it's believed this was caused by fumes," the airline said in a statement.

About 100 people were in the Qantas office at the time.

​A Qantas employee said they were evacuated from their 6th floor office around 10am.

No alarm sounded and she could not smell any gas, she told Stuff reporters at the scene.

CHRIS SKELTON/STUFF The building is a 16-storey commercial office tower, housing Qantas Airways and Countdown, among other companies.

A St John spokeswoman said those taken to hospital had symptoms such as chest pains, nausea, active vomiting, headaches and general unwellness.

They were deemed to be in a stable condition, with moderate injuries. Nobody lost consciousness.

Most patients were from level eight, Barnett said.

About 30 St John paramedics had set up a on-site triage area to treat and assessed more than 100 people.

Fire crews and the bomb squad gave the building the all clear by 2pm and it was left in the hands of the building manager.

ADAM JACOBSON/STUFF People were presenting with symptoms such as chest pains, nausea, vomiting, headaches and general unwellness.

Six levels were evacuated in total.

St John advised anyone who was in the building, who began to feel delated symptoms of nausea, vomiting, headache, sore throat or eyes to visit their GP or nearest medical clinic.

WHAT HAPPENED?

The smell originated on the 6th to 8th floor, which were the first floors to be evacuated. It then travelled to the 9th and 10th floors, which were also evacuated, Woon said.

Woon said as the smell had mostly dissipated by the time the fire crews arrived.

"Whatever happened has gone."

It is understood the air-conditioning unit, which feeds from the roof, was the source of the problem.

The air-conditioning system had since been shut down.

An email to tenants from Bayleys Property Services property manager Hamish Mackereth, sent after midday, said the leak came from an "isolated air conditioning incident" on level seven.

"We are aiming to have the air conditioning up and running again within the next couple of hours.

"Please note that if you do vacate from the 19 Victoria Street Lobby, authorities will not let you enter back into the building via this entrance."

After 3pm Mackereth said the building was back to business as usual, and the landlord would be working alongside the Auckland Regional Public Health Service and Worksafe to investigate the cause of the mystery gas leak.

In a statement to media, he said the source of the fumes had not yet been identified.

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

Woon said he was confident there were no fumes coming from the air-conditioning unit when it was assessed earlier on Thursday.

Augusta House is a 16-storey commercial office tower, housing Qantas Airways and Countdown, among other companies.

People inside the building were evacuated at different times throughout the morning, and were told by officials there were "no guarantees" they could re-enter the building.

They were taken to an assessment area "and if we were feeling OK, we were told to go to one area, and others were told to go to triage".

Worksafe confirmed it was notified of the incident.

PUBLIC SAFETY

Earlier, Woon said people would be allowed back into the building once results of the cause were conclusive.

Surrounding buildings had been assessed and there was no smell present, Woon said.

"We're pretty happy that the smell has gone . . . but we don't know what's going on."

It could be one of two things: there was never any gas there to start with, or there was gas there and it had dissipated, he said.

Countdown Metro store manager Ken Lawrence said though the supermarket was semi-attached to Augusta House, police advised him at 10.30am there was no danger to Countdown staff or customers.

Police partially cordoned off the street, and there was a heavy emergency service presence in the area.

Victoria Street reopened after 2.30pm and buses returned to their normal routes.