Work is due to begin within a month on "Project Big Splash", a high-tech water ride flowing through the ecological evolution of New Zealand.
Rainbow Springs project director Stewart Brown said the project also included construction of an outdoor 400-seat, free-flight bird show auditorium, an interactive children's playground and a new reptile enclosure.
The development, due to open early next year, was the largest single investment made in the park since it opened in 1932, he said.
Ngai Tahu Tourism chief executive John Thorburn said it was great news for Rainbow Springs and also the whole of Rotorua as a tourist destination.
"While the tourism market is ever changing and presents continual challenges, Ngai Tahu Tourism is investing for the long term. As one of the integral hearts of New Zealand tourism, with its strong culture and New Zealand history, Rotorua is a key focus for us."
