A security firm has apologised to Greens senator Sarah Hanson-Young for spying on her while she was visiting an immigration detention centre in Nauru in 2013.

Senator Hanson-Young was given the code name The Raven when at least one Wilson Security guard, acting without authority, kept her activities under surveillance at a hotel and outside the centre.

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Wilson security executive manager John Rogers offered an unreserved apology to the senator during a Senate inquiry hearing on Monday and condemned the “isolated actions of a misguided individual”.

Detention centre operator Transfield services also offered an apology.

“It was extremely regretful,” senior company executive Kate Munnings said.

Ms Munnings said it was usual corporate practice to have code names to protect the privacy of projects with high levels of confidentiality.

She maintained the raven code name was the decision of an individual who went rogue but insisted the actions were in the interest of the senator’s security.

The staffer lost their job and was subject to disciplinary action, the inquiry was told.

Ms Munnings repeatedly declined to confirm whether Transfield staffers were present at security meetings when the senator’s itinerary was discussed before her arrival.

Senator Hanson-Young questioned why she was not informed of the spying incident in a timely manner.

The inquiry was told the man who instructed two junior guards to spend their 12-hour shift watching the senator’s car in the hotel car park was suspended with pay once his actions came to light.

The pair had been instructed to employ a “ghosting technique”.

The supervisor was stood down from his role for a year and then later reappointed to a senior role.

No disciplinary action was taken against the guards.

Mr Rogers denied any listening devices were placed in the senator’s room or vehicle.

He said the anonymous author of a written submission to the inquiry that raised the spy allegation was a disaffected former employee.