The group proposes to protect "our sacred school on Puhitahi Hill", which is known for its high-profile alumni including Jonah Lomu, Temuera Morrison and Weta Workshop co-founder Sir Richard Taylor.
Mr Malcouronne represented the group at the Wesley College Old Students Association AGM on Sunday and spoke of their concerns at the extent of the plans and the lack of consultation with past students.
Mr Johnston said the group was in the minority compared with the extensive support for the project and securing the school's financial future would protect its special character.
He said fees had risen to $16,500 a year, a figure that was subsidised by the school down to $7500.
"The cost of running a boarding school is huge ... we are protecting the school forever by having these funds so that in 50 years the school still exists ... we won't be able to afford the rising costs otherwise."
Mr Johnston said the project was still in its early development stages and the board had not ruled out the possibility of relocating the school to another rural area.
Mr Malcouronne said financial gains could be achieved by developing just part of the school's farm.
"We could ... make pots and pots of money, far more than what's needed. Building a city around it makes it no longer Wesley College."