The ANZ could potentially lose 30 to 40 per cent of its National Bank customers by dropping the brand, a banking expert says.
But the bank says it has not had any "appreciable change" in customer numbers since it made the announcement last month and it plans to grow market share.
Massey University senior lecturer Claire Matthews said past bank brand mergers in New Zealand showed a loss of customers was likely, although it could take several years.
Matthews said Westpac experienced a large jump in its share of the home loan market after it absorbed the Trust Bank brand.
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