Mr Adcock said the council has received four reports of the letters being delivered so far. Two were from members of the public and two were from council staff members.
He said the council became aware of the issue on Thursday but cannot be more specific on the time frame of delivery.
Mr Adcock said the letter has the council logo on the top right hand side and includes council contact details.
He said the matter has been referred to Onerahi Police.
The council is asking anyone who receives one of these letters to enclose it in an envelope and drop it off at Whangarei District Council, with their name and address on the back and attention it to David Palmer.