In correspondence to Campbell, the Banking Ombudsman said the ANZ banker had not followed the bank's correct processes in its terms and conditions and had not recorded the meeting with the woman, Stuff reported.
In the final decision, Nicola Sladden of the Banking Ombudsman recommended that the bank reimburse legal fees incurred and pay $6000 in compensation for stress and inconvenience.
But Campbell and her mother's estate believed it should pay out $335,000 to the estate, Sladden said in the report.
"The bank has acknowledged that it should have obtained signatures before changing the account. It also apologised, accepting that its actions had caused unnecessary distress to Maria's family," Sladden said in documents seen by Stuff.
"In response to our preliminary view, the bank has agreed to pay total compensation of $18,755.94 in full and final settlement of the complaint."
Campbell has sought legal advice and said the family had not yet decided if they would accept the offer, Stuff reports.
The Herald has contacted ANZ for comment.
ANZ spokesman Stefan Herrick said the bank manager firmly believed he had authority to consolidate the dying woman's accounts.
"This is a case of a staff member trying to look after a long-time, loyal customer who was sick and bedridden, by following her wishes to visit her at her home and to close her individual accounts and transfer the balances to a joint account," Herrick told the Herald.
"The staff member had a long relationship with the customer and knew her well. We believe she was of sound mind and had no doubts about her ability to make decisions on her accounts. Unfortunately, the customer passed away before the mandate papers could be signed the next day. The staff member firmly believed he had the authority from the customer to consolidate her accounts, and was obligated to act on her instructions.
"This is fundamentally an intra-family dispute about the assets of a deceased member of that family, of which ANZ had no wishes to be drawn into.
"We have honoured the Banking Ombudsman's findings including apologising to the customer. Ms Campbell has yet to accept the Banking Ombudsman's ruling."