Five Australia Post employees have been taken to hospital after being exposed to an unknown chemical, believed to be acid, at a package sorting facility in Sydney's north. One worker suffered burns to his hands after a liquid spilled from a three-kilogram satchel that had been sent through the mail to the Frenchs Forest Delivery Centre just after 8am on Monday. He had been isolated in Manly Hospital until the liquid chemical was identified, the Fire and Rescue NSW spokesman said. The substance is believed to be acid, and police have launched an investigation into the incident. Australia Post spokeswoman Mel Ward said staff found the unidentified liquid in a parcel that they were processing at the delivery centre. "The safety of our staff and security of mail is of paramount importance to Australia Post," Ms Ward said. Staff at Frenchs Forest Delivery Centre found an unidentifiable liquid in a parcel they processed. Australia Post's standard emergency response procedures were immediately activated and emergency services called. The area was isolated and staff were evacuated from the building. Hazmat officers and police attended the scene to investigate. She said the Australia Post staff members were expected to be released from hospital soon.