Four leaky buildings will be demolished in the project, which is expected to be completed in time for the start of term one in 2020.
They will be replaced by a two-storey block containing 14 learning spaces and a library, and an adjoining single-storey block, housing administration and staff.
Another leaky building will be reroofed and its external cladding and windows replaced, while the school's hall will undergo minor repairs.
A concept master plan for the rebuild, originally completed by Dunedin company Logic Group in 2015, has been turned into plans by Christchurch firm Noordanus Architects.
Principal Chris Bryant said the rebuild would expand the school's capacity from 600 places to about 700.
Building work would be carried out in confined areas to minimise disruption to the rest of the school, Mr Bryant said.
Although the redevelopment was starting later than originally expected, the delay was due to the design being refined to get the best possible result while staying within budget.
Although the leaky buildings had never been a health hazard, they needed to be dealt with, and incurred higher maintenance costs.
He was pleased that issue, and the school's ability to cope with projected roll growth, would be resolved without encroaching on its green space.
The refurbishment of one leaky four-classroom block, completed in May, had effectively been the first stage of the rebuild, he said.
That building had shown how the rebuild would transform the school's learning spaces.
It was open and light, and "flexible and adaptive'', to modern learning needs.
