Meetings of the project control team set up for the district plan review were poorly attended because of heavy staff workloads. More specialist staff had now been appointed to it.
City environments general manager Brian Croad was also taking a more active and direct role in the process to avoid further mishaps.
The Herald has obtained an email from Mr Croad to staff across his eight areas highlighting "the adverse publicity" caused by "floodgate".
In the email Mr Croad said: "We should have handled this better and have learned a valuable lesson around communication and quality control."
His email has angered some frontline staff, who feel they were being unfairly blamed for something they had no control of.
A council worker responded to Mr Croad's email, saying many of the staff dealt with the public every day and were "well aware of reputation damage and cost control".
Mrs Hardaker was unaware of the emails but said staff she had spoken to had taken responsibility for the mistake.
"Staff were very concerned about what happened there and accept they should have taken better responsibility of those letters, and I accept that.
"Things happen, we have to learn from them so those sorts of things don't happen again."
The mayor was confident changes made to the council's process would address the issues and said city politicians would also be monitoring it.
"I along with the councillors took responsibility for what happened in the organisation overall."