A closed Christchurch worksite is being entered by chemical specialists after a worker was poisoned and hospitalised about two weeks ago.

An industrial storage unit that contained pest control agents is at the heart of the work now being undertaken by the Institute of Environmental Science and Research (ESR).

ESR has been asked by WorkSafe New Zealand to determine the concentrations of chemicals on site.

Fire and Emergency New Zealand is assisting near the corner of Wickham St and Francella St.

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A WorkSafe spokesman said testing today was designed to ensure the site is safe and to find whether the incident breached any health and safety or environmental laws.

"Hazardous substances specialists from Fire and Emergency New Zealand are on site to ensure it is safe for ESR technicians to enter to obtain samples," he said.

"These samples will provide further information about what is present on site to enable decisions to be made on how the site should be managed from now on."

The spokesman said all parties were taking a "highly precautionary approach" to today's activity.

Pest Control Research chief executive Matthew O'Brien said the company was fully cooperating with the official WorkSafe investigation.

O'Brien said the incident occurred on May 28 when a worker was processing chemicals.

"He appears to have come into contact with a poisonous substance," O'Brien said.

"He is in hospital and we are closely supporting him and his family."

O'Brien said PCR was unable to comment on the circumstances leading up to, or the causes of, the incident.

"Clearly that will be part of both the independent investigation we have commissioned, and WorkSafe's inquiry.

"We will be able to comment in due course when all the facts are known."

O'Brien said PCR had been operating for many years in Christchurch manufacturing pest control devices and poisons without incident.

"We are very experienced at handling dangerous chemicals and have an excellent safety record.

"Obviously, we are very concerned this incident has occurred and will do everything we can to find out how it happened, why, and what can be done to prevent it from happening again."