Staff and guests have returned to all levels of Sydney’s Pullman Hotel after caustic fumes swept through several levels of the building, sparking an evacuation.

NSW Ambulance crews were initially outnumbered as 30 people required treatment after being exposed to an odour from dangerous chemicals used in the roof-top pool.

“One of the workers has mixed two of the pool chemicals together inadvertently and there’s been a gas-off reaction to that,” Fire and Rescue NSW Duty Commissioner Charles Begley told 9News.

"The (chemical) drums aren't clearly marked so people do make mistakes," he said.

The view from the hotel's 22 floor, where the swimming pool is located and the chemicals were inadvertently mixed together. (Pullman Sydney Hyde Park)

Emergency workers treated up to 30 people at the hotel, with eight rushed to nearby hospitals. (9NEWS)

"Fortunately in domestic incidents it is far smaller quantities.

The top three levels of the luxury hotel were evacuated as fumes from the chemical reaction between hydrochloric acid and chlorine were swept into a nearby air conditioning unit and lift shafts.

Staff and guests who came in contact with the chemical suffered a mixture of respiratory and irritable eye complaints and were treated in the five-star hotel’s foyer.

In a statement to 9News, the hotel’s operators, Accor Hotels, said “as a precautionary measure, NSW Ambulance confirmed two guests, four permanent staff and two contract staff exposed to the irritant were taken to hospital. As at 3pm today, all guests have been discharged and one permanent staff member is still being assessed by doctors.”

Gas detection units were used to sweep through the affected levels and were deemed safe by emergency response crews.

A woman emerges from the hotel covering her face, after affected levels were evacuated. (9NEWS)

Up to 30 people are being treated after a hazardous chemical scare at the Pullman Sydney Hyde Park hotel. (9NEWS)

Auckland visitor Christina Sticovich said they saw the ambulance crews and staff at first thought it was a drill.

"All these people were sick and the ambulance people were checking them out, we were a bit shocked and surprised this was happening," she said.