The ceremony took place at a gala attended by present-day pupils and former pupils from around New Zealand and the world, more than one hundred years on from when the foundation stone was laid on June 27, 1912.
The school's youngest pupil, Natalia Atherton, and oldest surviving pupil, Sr Winifrede, marked the occasion by cutting the cake together.
Sr Winifrede said she still remembered refusing to stand with the rest of the juniors on her first day in 1918 - instead deciding that she wanted to stand with the school's senior pupils.
"A sister came over and said it's time you moved over to the juniors and I said 'I am not going over there I know everything'. She tried to lift me up and I gave her a mighty kick in the shins.
"I heard in later years that this nun that I kicked developed a very sore varicose vein," she said.
It was a "great privilege" being asked to cut the cake, she said.
Principal Richard Duffy said he was pleased with the turnout to the gala and said it was a great way to celebrate the history of St Mary's Church and School.
"It just shows the heart of the school and the heart of the community is strong here."
Attractions at the gala included a range of food stalls and a "kiddieland", with bouncy castles, face-painting and other activities for children.
The centenary was also marked with the release of the book 100 Years On The Hill 1912-2012.
