The scandal, in which the details of thousands of clients were sent to Ms Pullar, also claimed the scalps of ACC Minister Nick Smith, board chairman John Judge and directors John McCliskie and Rob Campbell.
The board made the payments due to Mr Stewart's willingness to stay on for three months after he'd resigned, Ms Melville said.
Mr Stewart worked at ACC for a year, with an annual salary of around $500,000.
ACC chairwoman Paula Rebstock said this week she would announce a successor to Mr Stewart next month.