"He resigned in October 2017, while the criminal investigation was still ongoing and before any possible employment investigation could be completed," de Wattignar said.
"Police commend the bravery of the victims who knew Vollmer's behaviour was wrong and came forward to make a complaint.
"There is a robust process in place to manage complaints about staff conduct. As evidenced by this prosecution, we do not hesitate to investigate and hold our people to account where appropriate."
Vollmer resigned in October 2017 while the criminal investigation was still ongoing, de Wattignar said, and before any possible employment investigation could be completed.
The acting district commander said the New Zealand Police expects high standards from its staff, reflective of their values.
"We are deeply disappointed in the actions of Vollmer, which do not align to the professionalism and integrity of the 13,000 men and women of New Zealand Police who come to work every day to prevent harm in our community," de Wattignar said.