The P2P specialist announced the hire this afternoon, but Stevens will not join the business until "early 2020".
Stevens is perhaps best known in the New Zealand market for his spell as managing director of FlexiGroup, the commercial and consumer lender. More recently he worked for medical payments startup MiFund.
Stevens has experience with corporate acquisitions, and while with FlexiGroup, spearheaded the takeover of Fisher & Paykel Finance.
Stevens said: "“The senior leadership has done a fantastic job driving Harmoney to this point, it is now a mature business and I can be useful as we take the business to the next level. The performance of Harmoney to date has been miles ahead of the Australian competition – what Neil and his team have developed is world-class."
His appointment will lead to a reshuffle at the top of the business, with the company's current joint CEOs stepping down from their roles.
Neil Roberts, Harmoney's founder and one of the joint CEOs, will focus on "strategy and product", the company said.
While Brad Hagstrom, the other current joint CEO, will take on the chief operating officer role. Hagstrom will be tasked with "supporting the growth of the platform", Harmoney said.
Harmoney has reached loan volumes of more than $1.2 billion, with $100 million of that in the Australian market.
The company said in a statement: "Mr Stevens enters the business in a high-growth phase; it recently moved into profitability and continues to innovate in and disrupt the traditional financial services industry in Australasia."