The Inland Revenue (IRD) error where babies and children received letters about their income tax last month cost taxpayers more than $82,000 and wasted more than half a tonne of paper.
The IRD apologised after the letters were received by puzzled parents, saying although most children are technically income earners for tax purposes, it was "a mistake".
"We shouldn't have sent any letters to children who are not income earners, technically or otherwise," a spokesperson said.
1 NEWS asked the IRD to provide the estimated cost of the mistake through an Official Information Act request, which has now been answered.
"Inland Revenue incorrectly sent letters to 118,592 people under the age of 18," IRD's Group Manager of Marketing and Communications said.
"The estimated cost of producing and mailing these letters was $82,765 (excluding GST).
"The letters were sent due to a manual error while selecting data from Inland Revenue's heritage IT system, not due to our new system."
A ream (package of 500 sheets) of 80gsm A4 paper weighs about 2.5kg, according to international standards.
With 118,592 letters sent in error, about 237 reams of paper - or more than half a tonne (592.5kg) - was wasted.
The above weight total does not include the envelopes the letters were sent in.