A woman sent a letter from IRD is shocked she also received mail meant for two other people.
The privacy breach is the latest in which several government departments have passed on personal details to unintended recipients.
The woman, who wishes to remain unnamed, says when she opened her child support bill, she was horrified to find two other letters along with names, addresses and details of one person owing $27,000 in child support.
She says she will return the letters.
IRD acting group manager of customer services, Denis McDermott, says such incidents are taken seriously and investigated but, regrettably, mistakes occasionally happen.
He apologised to anyone who may have been affected by the breach.