Purdey, who joined the dealer group in April last year is the second chief executive to resign from the group.
The first CEO, Dean McDougall, also resigned his position after about a year in the role.
Newpark chairman Burton Shipley has confirmed he is stepping down from the board.
"I was elected a Vice President of FIBA, the International Basketball organisation in August last year and my workload there has increased dramatically with a large amount of travel," he said.
"I began talking to fellow directors about retiring from Newpark last year and announced to the board at our January 2020 meeting that I would like to stand down and if possible the end of the financial year would suit me and Newpark well."
Good Returns understands a new appointment is due to be announced soon.
Purdey says: "I have been privileged to have learned so much from my tenure at Newpark, and most especially from the relationship we have started to build. The support and feedback from advisers has been humbling. I am pleased to know I have made a difference for so many," Purdey said.
"It has become clear to me that the vision the Board and I share for Newpark as to the quantum of change required in the areas of conduct and culture moving into the regulated future is disparate to that of the shareholder/s and meaningful influencers at Newpark.
"I will be looking to my future now as a passionate advocate for bringing quality trusted financial advice to New Zealanders," she said.
Shipley said "I am disappointed to see her resign but differences of opinion between boards and CEOs do occur and a way forward must be found. I wish Melanie well in whatever challenge she takes up."
Newpark appointed McDougall as its first chief executive after founder and general manager Darren Gannon said he would be stepping down from this leadership role in late 2017.
“I’m not retiring but I’m definitely handing over the reins," Gannon said at the time.
“After 32 years selling insurance and helping others to sell insurance means it’s time for a change of ideas, energy and people.”
“I’ve done everything I can, it’ s time for new blood,” he said.