"We also heard the army may be coming to clear the building but we don't know how official this update is."
A police spokesperson said the threat was made against "a Waterloo Quay address" and asked members of the public to avoid the area.
Reporter Jack Crossland, who was on the scene at 10am, said most workers had dispersed into cafes.
"There's about six police cars on the scene at the moment, there's a few police outside and I imagine there will be some inside as well."
He said the PWC building and neighbouring apartment block had been cordoned off, and the bomb squad were scouring the building and outside area "making sure there's no stone left unturned".