Darby is presenting a $200-a-ticket stand-up comedy show for the Wanaka Primary School Future Trust, which is raising money for school resources.
The ticket price includes a four-course meal and wine.
Trust chairwoman Sandy Guy said securing Darby for the fundraiser was an "unbelievable coup".
She traced him on the internet, but before Darby would commit, Ms Guy had to convince his wife, Rosie Carnahan, she was not a crackpot, she said.
She was able to do that by referring Darby and Carnahan to the charitable trust's website.
"I can't believe they replied.
I thought, this is a really longshot," Ms Guy said.
She then spent the best part of seven months chasing him for a date and was nearly ready to give up.
Then, Darby found he could slot the Wanaka show into his family holiday.
He will appear at the Queenstown Winter Festival after his Wanaka performance.
Darby (34) is one of the most sought-after acts in New Zealand, following a successful international comedy and acting career, which he is still building.
He played Norman in the Warner Bros film Yes Man (starring Jim Carrey) last year.
This year, he is Angus in Working Title's British film The Boat That Rocked.
And he is still playing inept band manager Murray Hewitt on the Flight of the Conchords HBO television series in the US.
Tickets for the fundraiser, to be held at the Lake Wanaka Centre, went on sale yesterday.
