Fumes at Syd pool sends kids to hospital

Four children and two adults have been sent to hospital after chemicals at a southwest Sydney swimming pool were mixed incorrectly, creating a poison gas cloud.

One child is in a serious condition and several others have been hospitalised after they were overcome by chemical fumes at a Sydney swimming pool.

Emergency services were called to Wollondilly Community Leisure Centre in Picton, in the city's southwest, about 10am on Monday after an incorrect mix of chemicals caused a reaction.

Nine children under six years of age and two adults were treated by paramedics for breathing difficulties.

One child was taken to Campbelltown Hospital in a serious condition while three other children and two adults were hospitalised in a stable condition.

It's understood that dry chlorine and sodium bisulphate were pre-mixed and added to the pool, forming a gas cloud, a Fire and Rescue NSW spokesman told AAP.

Sodium bisulphate is a dry acid commonly found in cleaning products and is used to lower the pH balance in swimming pools.

Crews ventilated the area and the gas dissipated shortly after.

Wollondilly Shire Council said there had been an "operational need" to use a "super dose" of disinfectant in the indoor pool on Monday morning.

"Unfortunately a chemical spill occurred during this process, resulting in fumes," the council said on its Facebook page.

The indoor pool of the leisure centre will remain closed until mid-afternoon as a precaution.