Chemical scare at Sydney Water building

Hundreds of workers have been evacuated from Sydney Water's headquarters in Parramatta after a chemical scare.

The alarm was raised at a tower block just before 1pm (AEST) on Monday, with police sealing off parts of the road and forcing staff to abandon their desks and wait outside for much of the afternoon.

Fire and Rescue NSW said chemicals had mixed in the basement and the compound was giving off a strong, potentially dangerous vapour.

Firefighters wearing protective suits and rubber gloves entered the basement to try to identify if there was any imminent threat.

It is understood they isolated one of the chemicals in a 50-litre drum and made it safe.

One firefighter was seen being washed down with a hose outside the building.

Atmospheric testing was also being conducted in and around the basement, which was ventilated with fans.

Staff late on Monday afternoon told AAP they expected to be allowed back into the office soon and said nobody would be allowed into the basement until at least after 6pm.