Private security guards at the Australian-run detention camp on Nauru allegedly took the extraordinary step of running a secret surveillance operation on an Australian senator.

A former guard has alleged Greens senator Sarah Hanson-Young was given the codename "Raven" and covertly followed and her hotel watched when visiting the asylum-seeker detention centre on the Pacific island nation.

Greens Senator Sarah Hanson-Young. Credit:Alex Ellinghausen

Wilson Security – the company contracted to guard the detainees – has rejected the claim a team was organised to spy on Senator Hanson-Young during the December 2013 visit but admits "individuals" have been disciplined for "acting beyond their brief".

The security company has said the primary motivation was security for the senator but the activity was "not authorised".