"I continue to be constantly amazed by the generosity of people in our local communities," he said.
Geyser's current chairman Kierin Irvine said trustees were delighted that La Grouw had accepted the role as the foundation's inaugural patron.
"The role of patron is honorary, and we are very grateful that Joe has made this generous commitment as he brings a great deal of business and governance experience to the position."
In acknowledging the enormous contribution La Grouw had made to the foundation, Dr Irvine said, "Joe, together with his wife, Jo-Anne have always led by example. They established Geyser's first endowment fund, which will benefit the local community for generations to come, and generously contributed to the foundation's operating costs over many years.
"Joe's knowledge of and dedication to his local community helped introduce the community foundation concept to people in the Rotorua and Taupo Districts, and ensured we remain in touch with the communities we serve.
"As patron, he will be a guardian of our values, bring an independent view from the community to the board, have mana and judgement, and be available to the public," Irvine said.
Thanks for the generosity of a growing number of local people, Geyser has more than $5m invested and $14m anticipated in people's wills.