A woman who was hit by a bus in Sydney's CBD and trapped for two hours on Wednesday afternoon has died.

The woman, 53, was taken to Royal North Shore Hospital with multiple critical injuries after the incident on Clarence Street about 3.50pm.

However, police said the woman died several hours after she arrived at hospital.

Office worker John Preston said he was eating a pie with a colleague when he saw the Sydney Buses vehicle run over the woman as it turned into Clarence Street from Erskine Street.