Otago Cricket Chief Executive Mike Coggan said King decided it was appropriate he stepped down from the role even as the association worked through a thorough season review.
"Nathan has been an outstanding servant of Otago Cricket working previously as a development officer and recently as the Volts assistant coach, " said Mr Coggan.
"I certainly do understand just how difficult this decision has been for him."
King said his resignation "was in the best interests of both Otago Cricket and his family. I have throughly enjoyed the challenging but also rewarding aspects of coaching the Otago Volts".
"I would like to thank Otago Cricket for the opportunity and wish them, along with all the players the very best for the future. Coaching is still something I remain very passionate about and would love to stay involved in some capacity in the future," he said.
Otago Cricket was unable to comment further due to the ongoing review process and confidentiality agreements.
