The ERA also found Mr Simpson left himself open to bias criticism, made an untrue statement, conducted a "sham" interview and used double standards during the dismissal process.
Any action the council took over the ERA ruling remains unclear.
In September, the WDC revealed that its cost for responding to Mrs Walters-Gleeson's claim to the ERA came to $184,609.12, including GST.
The cost of an earlier internal council review of her dismissal came to $9059, including GST.
Mr Simpson earned $299,000 in the 2013 to 2014 financial year.
Whangarei Mayor Sheryl Mai thanked Mr Simpson for almost two decades of outstanding service to the district.
"Mark's contract with council has expired and council is now in the process of negotiating a contract with a new CEO. The council and I extend our best wishes to Mark. Under his stewardship this organisation and the district as a whole has made enormous progress," Ms Mai said.
"Mark began at council in November 1996 as Waste and Drainage Manager. In October 1998 he was appointed CEO and has held that position ever since."
She said in the 2000s Mr Simpson was responsible for setting a mission statement for the council, Creating the Ultimate Living Environment, and identifying values for the organisation.
"He led us through the Business Excellence System, and later envisioned "Whangarei at a Crossroads". One of the most outstanding achievements by the organisation under his leadership was completion of the Lower Hatea Crossing, Te Matau a Pohe. This project was one of the final links in a roading capital works programme conceived of in the mid-1990s."