"They're understandably devastated, in shock, about what this will mean for them and their livelihoods, and those in their communities.
Williams said many communities will be put out if the closures go ahead.
"The next branch along is often about an hour away and that's going to be a problem, not just for staff but for customers too."
A Westpac spokesperson said they are continually reviewing how to best meet their customers' evolving needs.
Online transactions had increased by 61 per cent over the last five years, and logins via mobile devices went up 33 per cent, they said.
Staff had been asked for feedback over the proposals.
Branches that will close are:
Kamo, Whangarei
Broadway Avenue, Palmerston North
Bishopdale, Christchurch
Cherrywood, Tauranga
Gardens, Dunedin
Waikanae
Carterton
Raglan
Ranfurly
Te Aroha
Fendalton Mall Christchurch
Putaruru
Whangamata
Takaka
Stoke
Wainuiomata
Te Anau
Fairlie
Otorohanga