Lynch said rather than appointing a new trustee or manager the NZACU decided to outsource all KiwiSaver responsibilities.
"We didn't want to be a [KiwiSaver] promoter," he said.
According to Lynch, the NZACU wanted a New Zealand-owned KiwiSaver partner, which put Fisher in the front-running position.
"We also needed a KiwiSaver partner that wasn't a bank or insurance company," he said.
The deal also leaves default provider Mercer, which formerly acted as the KiwiBank scheme provider, without any third-party retail distribution for its main KiwiSaver scheme.
Martin Lewington, head of Mercer NZ, said it would continue to receive members through the KiwiSaver default scheme allocation process.
"We're also looking at ways to engage employers to grow [the Mercer Super Trust KiwiSaver scheme]," Lewington said. "Our natural turf is b-to-b [business-to-business]."
The credit union KiwiSaver scheme has approximately 4,200 members. However, Lynch said with 170,000 credit union clients nationwide there was great scope for growth in its new arrangement.
Under the terms of the deal, Fisher would not be permitted to market directly to credit union members.
Carmel Fisher, head of Fisher Funds, said final sign-off for the credit union KiwiSaver transfer was probably three months away.
"But we're already training up credit union staff on our scheme," Fisher said.
Fisher has been an active acquirer in the KiwiSaver space, paying $20 million to buy the controversial Huljich scheme last year.
"This is a scale game," she said.