Instead, the ministry spent about $7m on strengthening it before work ground to a halt in the last two years, due to cost blowouts and disruption to classes.
The ministry had told RNZ it spent $5.4m on construction, however this did not take into account another $2.1m of other costs connected to the main block and revealed in an OIA response.
In a newsletter to families, principal Christine O'Neill said the ministry had confirmed a rebuild last week.
"New buildings will deliver us the best long-term facilities and minimise disruption to teaching and learning," she said.
"We have been in regular contact with the ministry through this process and fully support this solution."
Former board chair Mike Lay earlier accused the ministry of covering up a waste of taxpayers' money and time spent pursuing the wrong option against expert advice, and retaliating against him when he questioned officials.
The school's technology block will also be replaced, and an art block refurbished.
Construction is likely to start in late 2022, just prior to when the whole city-wide school rebuild programme was meant to wrap up in 2023.
"There will be significant planning and design work that needs to take place before we get to the construction phase," O'Neill said.
"We will be working with the ministry in the new year to identify a design team for the project and we will work closely with both them and the ministry to ensure we deliver the best possible outcome for our school and community."
The ministry's Education Infrastructure Service head Kim Shannon in a statement said: "At the recent meeting with Christchurch Girls High School Board of Trustees, it was decided that we will build a new main block in a different location on the school site.
"We are also building a new Technology Block and refurbishing the Arts Block. The location of these two new buildings will be less prone to liquefaction and potential flooding effects from the Avon River.
"We'll work with the school over the coming months to determine the best location for the new buildings, as well as carrying out detailed assessments of ground conditions, building design and other inputs needed to ensure we are delivering the best solution.
"The main block will continue to be used by staff and students while the new facilities are being designed and built. We expect to move into the design process for the new facilities next year.,
"Investing in strengthening the main block was assessed as the best option at the time.
"The OAG looked at the decision-making process as part of its investigation into concerns raised with the planning and management of building works at CGHS."
- RNZ
