A mystery gas leak in a Hamilton building has put two people in Waikato Hospital with another 12 people needing to be decontaminated.

Fire services were notified around 5pm to a smell "like LPG" in the ProPharma building on Aztec Place, Waikato area commander Roy Breeze said.

"When fire and ambulance arrived there were a number of people coming out of the building and one collapsed and was unconscious - [she was] not in a good state," Breeze said.

The woman was treated with oxygen and she and another patient were decontaminated and taken to Waikato Hospital. A further 12 people were decontaminated with showers and given fresh clothes, and neighbouring buildings were evacuated.

Advertisement

Breeze said fire services were still unsure what the leak was and whether it was chemical or flammable.

"We still don't know what caused it but the smell has gone away," he said. "The challenge is that whatever it was that leaked has done its dash so we may not find anything."

Firefighters in breathing apparatus and with gas and chemical detectors were going through ProPharma and the surrounding buildings trying to find the leak.

If it is not found the building will be ventilated, locked up and checked again tomorrow.

Employees from the building have been sent home with advice on what to do if they begin to feel sick.

"All we know is that other people are not showing signs of sickness and the status two person is getting better so that just means that whatever it was isn't going to cause long-term damage," Breeze said.