City of Sydney CEO Monica Barone has been forced to apologise after sending a highly confidential letter threatening to discipline a councillor to the wrong person.

But the extremely private letter in which she outlined the pending disciplinary action was sent to Potts Point & Kings Cross Heritage Conservation Society president Andrew Woodhouse, who has very regular dealings with the council and read the letter on the assumption it was for him.

He was outraged by the mistake and has made a formal complaint to the local government minister.

Mr Woodhouse said: “In my 20 years following council machinations I’ve never seen anything so unprofessional,” and also defended Mr Mandla’s right to publicly raise concerns about council staff.