Ms Agnew said she had been inspired to write Super Finn by her students.
"Being around children every day I wanted to reflect some of their lives back to them."
"It's a book for them, about them," she said.
The night's top prize was awarded to Matt Elliott for his book Nice Day for a War.
The book, which was illustrated by Chris Slane, is about a World War I Kiwi soldier and is based on the original diaries of Mr Elliott's grandfather, Corporal Cyril Elliott.
Mr Elliott said many young children had been particularly interested in the period his book covered.
"It's obviously a period of young New Zealand history that they are really interested in, and they want to know more about what their relations did ... in the First World War."
He said it was an honour to write the book about his grandfather, who was "just an average Kiwi soldier" during the war. Other winners for the night included picture book, The Cat's Pyjamas, which won the Children's Choice award.
The book was written and illustrated by Catherine Foreman. Its main character seemed to be a crowd favourite among New Zealand children with his colourful collection of pyjamas.
The Young Adult Fiction category award went to Calling the Gods by Jack Lasenby and the Picture Book category was won by Rahui, which was written by Chris Szekely and illustrated by Malcolm Ross.
The book has been published in te reo Maori and English.
WINNERS
The Book of the Year winner for 2012.
New Zealand Post Children's Book of the Year
Nice Day for a War by Matt Elliott and illustrated by Chris Slane (HarperCollins Publishers).
Children's Choice Award
The Cat's Pyjamas by Catherine Foreman (Scholastic New Zealand).
Best First Book Award
Super Finn by Leonie Agnew (Scholastic New Zealand).
Picture Book category
Rahui (Maori version) by Chris Szekely translated by Brian Morris and illustrated by Malcolm Ross (Huia Publishers).
Rahui (English version) by Chris Szekely and illustrated by Malcolm Ross (Huia Publishers).
