Brown said there weren't any staff still at the school who were there when Karunatilaka was a student.
The author was in Porritt and Harvey House.
Brown said Karunatilaka's achievement was a great one for current students to follow.
"The power of the pen is always mightier than the sword. Keep writing, Keep reading ... leaders are readers."
Another former Whanganui Collegiate student, Simon Cairncross, said he remembered Karunatilaka being at school at the same time as him.
"This one is a pretty impressive prize for a Whanganui-educated person."
Cairncross said he was a big fan of, and recommended, Karunatilaka's book Chinaman: The Legend of Pradeep Mathew.
He said he had not had the chance to get his hands on The Seven Moons of Maali Almeida as it had only been published on August 4.
Paige's Book Gallery owner Lesley Stead was delighted with the news.
"Congratulations to Shehan and also to Whanganui Collegiate.
"It's a tremendous acknowledgement for Whanganui and the school. We have a strong literary tradition here which Shehan has now strengthened."
The Seven Moons of Maali Almeida follows the story of Maali Almeida, a war photographer who has woken up in the afterlife and tries to find out who killed him, all in the background of the Sri Lankan civil war.