While NZACU was did not directly compete with Co-op Bank, its members did.
No one else in the financial services sector had traded under "co-operative" or included in "co-operative" in their brand since Co-Op Bank changed its name from PSIS in 2011, Kennedy said this morning.
Co-op Bank had spent $8.7 million on marketing since then, he told the court.
This included recent television adverts of members celebrating the bank making a profit, which Kennedy said had successfully struck "a chord with the public".
"As a consequence the plaintiff has generated reputational goodwill," he told the judge.
Kennedy said his client was not alleging or claiming the monopoly of "co-operative" as a word.
"The plaintiff would not be concerned with the use of cooperative in a branding sense if was attached to sufficient other words that made it clear there was not going to be misrepresentation or confusion with the plaintiff's trading name."
The hearing continues today.
The NZACU, which represents 22 co-operatively owned credit unions and building societies, wanted to make the change from mid-July.
Co-op Bank earlier this year reported a net profit for the 12 months to March 31 of $7.1 million, up from $5.7 million during the previous period.