The 1999 Bloomsbury-printed book has special features which set it apart from newer editions, such as dropped text on one page and J K Rowling listed as Joanne Rowling in the copyright section.
The Estate at Webb's specialist Charlotte Dippie said this was what made it sought-after in the already highly collectable world of Pottermania.
She believed this was one of roughly 2500 first edition Harry Potter and the Prisoner of Azkaban books available.
"Harry Potter is such a dedicated fanbase. There is all kinds of collectors all over the world [for] who this would be quite a dream item," told Midday Report.
The unidentified New Zealand vendor's mother is said to have purchased the book in 1999, around the same time the fantasy children's series was gaining attention.
"I didn't read many books as a kid ... and this must have been the first one. I did form an emotional attachment to it," the vendor said.
Harry Potter first edition books have proven to have big value in global auction markets, with a Harry Potter and the Philosopher's Stone first edition sold for £21,000 (NZ$46,000) last week.
