"It then appears a member of the public secured the front doors of the bank to prevent the offender escaping."
Mr Hughes said armed officers arrived on the scene quickly after receiving several 111 calls.
There was no evidence the man intended to rob the bank, he said.
"While such incidents in Whangamata a rare and likely to attract widespread attention it is important to note there is no information to suggest this incident was in any way linked to a robbery and that emergency plans established for such situations worked well."
A Waikato police spokesman said the 53-year-old suspect was still being interviewed by police and no charges had been laid.
Police said victim support officers had been called in to offer support to the bank staff who were left upset and shaken by the incident.
BNZ spokeswoman Emily Davies said the man did not target or threaten any of the staff or customers inside the store.
They had evacuated while the man was carrying out his assault, she said.
"The store is currently a crime scene."
- Bay of Plenty Times and nzherald.co.nz