Five New Plymouth office staff were hospitalised with carbon monoxide poisoning after a gas leak in July.

A WorkSafe investigation is underway after five office staff in downtown New Plymouth were hospitalised with carbon monoxide poisoning from a suspected faulty work site generator.

The incident in July occurred in a Currie St building occupied by MediaWorks Radio, and the Public Trust.

Five people were admitted to the hospital's emergency department on July 1 with suspected carbon monoxide poisoning.

Taranaki District Health Board acting chief operating officer Wendy Langlands said the five patients were all were assessed and treated at the emergency department, and discharged on the same day.

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Patients treated for carbon monoxide poisoning were blood tested to ascertain levels of the poisonous gas in the bloodstream, and also treated with oxygen, Langlands said.

The gas leakage incident was confirmed by the New Plymouth manager of the Public Trust office.

Retail staff in an adjoining rebuilding said they were aware of the leak but were not affected.

A WorkSafe spokeswoman said the agency had been notified of the incident.

In an emailed statement a spokeswoman for MediaWorks said two team members were treated but were now well.

"There was an incident involving an on site generator at the site leased by MediaWorks in Taranaki," she said.

"More FM breakfast hosts were immediately taken off air, the building evacuated and the fire services were called.

"There is no damage to the building."

The company was conducting a "robust" investigation into what occurred and the incident has been reported to WorkSafe NZ, and the building's landlord was helping the company with the investigation, she said.