NZCU Baywide chairman Iain Taylor said the merger benefited both parties and gave Baywide "significant opportunity to grow the business within the Hawke's Bay health sector".
"Importantly, the merger with NZCU Baywide will mean our culture and philosophy of being member-owned and operated is retained," Ms Priest said.
The merger will become official in February.
Credit unions rank collectively among the country's largest financial service providers (by financial transactions), with one of the largest ATM networks in the country, outside of major banks.
Baywide merged with Manchester Unity Credit Union last year, boosting member numbers to 40,000. It announced a $1.2 million profit to June, a small increase on last year. Smaller credit unions have proven vulnerable. Hibernian Credit Union in Wellington was forced into liquidation after an employee stole $1.2 million over seven years.