Ngai Tahu Maori Rock Art Trust curator Amanda Symon said the collapse was the result of the "incredibly heavy rain" that had been inundating the area.
The site, located next to State Highway 83, will remain closed for the foreseeable future.
It is among the country's best-known Maori rock art sites and is a popular tourist stop, attracting an estimated 20,000 to 30,000 visitors a year.
Public safety was the main concern and the site would be closed until an assessment could be done on its stability.
"In terms of public safety, we can't have any risk," Ms Symon said.
Last year, a $260,000 car park was built to make the site a safer place to visit.
The closure was sad given it was a valuable site in terms of providing public education, but "at the end of the day, we want people educated, not squashed", she said.
It was a major landscape feature and an "amazing" site in terms of the rock art and the historical context it provided.
Many different styles and techniques had been used at the site and the wealth of red figures made it quite unusual.
The site was the first recorded instance of Maori rock art, noted in 1852 by the surveyor Walter Mantell.
The first photograph of rock art was taken there in 1896.
Some rock art pieces were removed from the site in 1916.
The collapse crushed part of the security structure surrounding the art and destroyed landscaping and fencing.
Some minor repairs had since been done to the caging.
The trust's main concern was making sure there was clear signage to ensure people did not enter the site, Ms Symon said.
