Fire officers have returned this morning to investigate a chemical smell at a Hamilton building where a gas leak put two people in Waikato Hospital last night - and have found the source of the smell.

Station Officer Dave Gunn said a hydraulic drill from a neighbouring premise was leaking.

He described the smell as like "rotten gearbox oil".

Workers at ProPharma have been briefed and are waiting to go back into the building.

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Firefighters will now hand over the scene to WorkSafe.

The Fire Service was called about 6.30am to the smell of chemicals at the ProPharma building on Aztec Place, where a mystery gas leak last night put two people in Waikato Hospital and another 12 people needed to be decontaminated.

Fire services were notified about 5pm to a smell "like LPG" in the building, 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 in showers and given fresh clothes. Neighbouring buildings were evacuated.

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