Fire crews are back at the scene of a suspected gas leak, which resulted in the hospitalisation of 12 workers from a central Auckland office building earlier today.

It's understood another two people have been taken to hospital afer falling sick.

An investigation is underway to get to the bottom of what caused the leak earlier today.

The patients were rushed to Auckland Hospital complaining of nausea and shortness of breath after four floors of the Augusta Building were evacuated around 9am.

Some of the patients were also vomiting following the incident.

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

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

St John Ambulance were also aware of a potential second incident.

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 defence force had not identified the source of the fumes, 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 has been cleared for re-entry and tenants have re-occupied the building.

The spokesman said the building's management team would provide an update when further

Fire and Emergency's assistant area commander Dave Woon said the building has since fully reopened.

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

"From our perspective, [we're] very happy the building's safe, so there's not much more we can do.

"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 uniformity of 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."