Operations manager Margot McCool said it was humbling to witness such generosity.
"We are incredibly excited about it, because these book awards will have assured funding."
Ms McCool said the donor had asked for privacy.
"It's someone who really wants to see New Zealand authors supported, someone who is really keen, who understands that the industry is a very tough one to make it as an author in New Zealand.
"Many struggle with families, work and everything else," Ms McCool said. "This award will let someone take a year out to write."
The awards are the country's premier honours for authors and 2016's event will take place during the Auckland Writers Festival, the country's largest literary gathering.
"We have fantastic authors in this country but to be able to make a living by being a writer they really need to be recognised internationally, and the book awards are obviously the best awards to do that."
The funding will be adjusted to match inflation over the years.
New Zealand Book Awards Trust chair Nicola Legat said she was delighted to announce the changes, in particular the major fiction award provided by the Acorn Foundation through the generosity of the donor.
"It creates a tremendous and lasting literary legacy. The sum of $50,000 will be awarded to the top fiction work annually, in perpetuity. This will make a difference not only to the receiving writer but also to the literary fabric of New Zealand. It is a huge gift for us all."
In addition to an annual fiction winner, there will be a poetry, a general non-fiction and an illustrated non-fiction winner and, should there be sufficient entries, a Maori language award.