Westpac Bank is looking to close a number of branches around the country.
The bank confirmed a "consultation" was underway with staff and First Union said up to 19 branches could be closed.
A statement from the bank said the rise in online transactions was behind the proposal.
"Currently more than 85 per cent of service transactions with us take place outside of a branch."
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The bank wouldn't confirm the number of branches that could be closing, saying "a number of branches" were included in the proposal.
First Union said the move puts up to 100 jobs at risk, although there could be some redeployments.
A decision would be made once feedback on the proposal from staff had been evaluated, the bank's statement said.
"Staff and customers will be the first to know that decision and there will be no further public comment until then," the statement said.
First Union said the branches included in the proposal to close were mostly rural and in small communities.
According to First Union the branches that could be closed were: Kamo, Whangarei; Broadway Avenue, Palmerston North; Bishopdale, Christchurch; Gardens, Dunedin; Waikanae; Carterton; Raglan; Ranfurly; Te Aroha; Fendalton Mall, Christchurch; Putaruru; Whangamata; Takaka; Stoke; Wainuiomata; Te Anau; Fairlie; Otorohanga; Cherrywood, Tauranga.