"The information that was shared was that of the outstanding rates total, and the address to which it relates. The email did not include your name or contact details."
The council has already contacted the recipients of the wrong rates bill reminders to request they delete the email.
Wellington City Council spokesman Richard MacLean said the situation was embarrassing.
He said the council was made aware of the issue soon after the reminders were sent out on Monday.
"The council has apologised to all these people and of course we're putting in steps to make sure it doesn't happen again. It would be safe to say it was human error.
"The phones haven't been running hot so hopefully people have taken our apology and understood the situation."
New safeguards will be put in place, including tightening review procedures.
There will also be an additional step in the process preventing one council officer from being able to complete the merger and preparation of these letters without review and approval. All officers in the team are being coached in this process.
The council has contacted the Office of the Privacy Commissioner about the breach.
MacLean said so far there have been no issues with people wanting late fees waived after not paying their rates because the reminder never reached them.
"But if someone comes to us and says they've been unfairly charged an extra 10 per cent, we will obviously look at each case on its merits."
