Health Services Union head Michael Williamson was stopped allegedly trying to remove documents as police raided his Sydney offices this morning.

Mr Williamson was stopped in the car park of the union's East branch building in Pitt Street, Sydney.

"Sad day for the organisation" ... Kathy Jackson. Credit:Peter Rae

Detective Superintendent Col Dyson, Commander of the NSW Fraud and Cybercrime Squad, said that any interference may result in criminal charges.

Up to 10 police officers raided the building to examine computers and seize documents.