Design flaws are believed to be responsible for leaks in the six-year-old $4 million Burnside High School art and technology block.
Similar leaks have also been found in the school administration buildings.
They will be repaired as part of a $24 million repair and redevelopment work plan at the school, which is set to start this year.
The cost of fixing the leaks is not yet known, but is expected to be assessed in the next few weeks.
The building has been leaking for more than two years, but work to fix it was put on hold while the Ministry of Education assessed other earthquake repairs and redevelopment work at the school.
Burnside principal Phil Holstein said the repair was initially not expected to be done until 2022.
It was brought forward to this year, as the school was worried damage would be done to the building if the leaks were not repaired, he said.
He said the architectural design of the buildings was believed to be responsible for the leaks.
“Across the country, a lot of school designs with this style of architecture, particularly flat roof buildings, a lot of them have weather tightness issues,” he said.
Alongside the repair work, he said the school was set to be refurbished and modernised.
“We want to provide the best learning environment we can for our students here, and that’s ultimately what we’re after,” he said.
Bringing the work forward meant the school could start the master-planning process with the Ministry of Education soon, he said.
Ministry of Education head of the education infrastructure service Kim Shannon said the work would start in the next few weeks.
Once the design work was done on the buildings, the cost of fixing the leaking issues could be assessed, she said.
“A designer will be appointed in the next few weeks to determine specifically what is required to fix the weather-tightness issues,” she said.
She said the leaks were the result of “historic design and construction work”, but who was liable was complicated.
“Proving liability for weather-tightness issues is not straight forward in Christchurch due to the wide ranging impacts of the earthquakes,” she said.
The ministry’s focus was putting the issues right, she said.