The Australian-owned bank will close its branches at the Gardens, in North Dunedin, Ranfurly and Te Anau under proposed changes announced to staff on Wednesday night, First Union national organiser Tali Williams said.
The two Otago branches and one in Southland were among 19 marked for closure across the country.
More than 70 jobs are at stake.
Three staff were employed at the Gardens branch and two at the Ranfurly branch — the town’s only bank — and there were no plans to relocate the jobs elsewhere, Ms Williams said.
"We strongly oppose all of these closures," she said.
"These communities need these branches.
"A lot of these are isolated and it could take up to an hour for people to get to an alternative branch."
There was "no pressing need" to close the branches.
Westpac New Zealand announced a $916 million profit for the year ending September 2015, she said.
"They can afford to keep Kiwis in work in these rural areas and they should," she said.
"People are in shock and really upset and don’t know what their future will hold.
"What Westpac might not say outright, but we know to be the case, is these towns don’t offer enough opportunities for debt sales," Ms Williams said.
The inability to generate greater life insurance, credit card and mortgage sales were the reason for their demise, she said.
A Westpac spokeswoman said the proposal was driven by changing customer behaviour.
"Currently, more than 85% of service transactions with us take place outside of a branch.
"A proposal regarding a number of branches is currently with staff for their consideration and feedback.
"Once that feedback has been received and evaluated a decision will be made. Staff and customers will be the first to know that decision and there will be no further public comment until then," the spokeswoman said.
First Union Dunedin organiser Shirley Walthew said Gardens staff found the proposal "hugely disappointing".
"We want to keep our members in work and their preference is for them to be open out at the Gardens," she said.
Westpac considered closing the branch in 2007, but backtracked following opposition from local businesses and residents.
Ms Williams said staff had been given two weeks to comment on the proposal and it was expected the bank would take a further four weeks to make its decision.
"We are really encouraging communities to come forward if they aren’t happy about these closures and let Westpac know," she said.
