The new building, dubbed "Augusta 2015", will cost $6.4 million.
The Ministry of Education will contribute $2.4 million, and the school has kicked-off the biggest fundraising campaign in its history to make up the difference.
Grammar wanted to go beyond the standard building government funding would provide, Mr O'Connor said, and he was sure its community would support that.
He was confident the new modern building would fit in practically and aesthetically with the school's five historic buildings.
It will replace pre-fab classrooms sited at the far end of the school on the corner of Mountain Rd and Clive Rd.
"We have worked with the Historic Places [Trust] in terms of how this building will sit on our campus, we are very conscious that we really are a conservation site.
"Hopefully as you come on site you will get a bit of a theme with our buildings, but it won't compete with the main block."
Grammar's changing face
• New three-storey classroom block to transform Auckland Grammar School campus.
• 12 classrooms and three teaching resource rooms to cost $6.4 million.
• School launches biggest fundraiser in its history to raise $4 million, with Ministry of Education funds making up the rest.