ANZ National has about two million customers, 9000 staff and almost 300 branches.
Mr Hisco would not say how many were customers of National.
He said the only difference they would notice in their internet banking would be the change of colour scheme, from green to blue.
Bank cards would be changed when they expired and security pins and other details would stay the same.
He said the company had spent two years ensuring systems from both banks were the same. Although 15 branches of the combined bank would close, there would be new branches in 15 extra communities.
When National and ANZ branches were very close, they would combine in the larger branch, or one would move to a nearby area with demand.
The bank workers' union said ANZ must make good on its assurances that jobs and services would not be cut as a result of the decision. FIRST Union said ANZ's "worst-kept secret" came as both brands were doing well.
Professor David Lont, from Otago University's accountancy and finance department, said the merger signalled the end of an "iconic" New Zealand firm.
The changes
* 15 branches will be closed, leaving about 280
* Branches will open in 15 extra communities - the banks will be in 217 towns and cities
* 9000 staff employed in both banks
* 2m customers in both banks
* The deal to use National's black horse logo, leased from Lloyd's in Britain, ends in 2014
* National Bank to be phased out over next two years