He had been told A-Line scored "way above" the other companies shortlisted in the tender process. The three other firms are understood to be from Auckland and Whangarei.
The project included about 21 new classrooms and a combined hall and gym. The only part of the current school to be retained would be the administration block.
The new buildings will cater for roll growth of 100 over the next three years.
The school has military, agriculture and hospitality academies, with forestry and manuka honey academies planned and a teen parent unit due to open next year.
A sod-turning ceremony was scheduled for February 4 with the new buildings due to open in early 2017.
The old buildings would be demolished or where possible re-used, Mr Rintoul said.
Andrew Abercrombie, senior project manager at A-Line Construction, said the company was very pleased to win the contract.
He believed the firm's 22-year involvement in education projects had put it in good stead. Local subcontractors would be used as much as possible.
The company is working on a $5m rebuild at Horahora School in Whangarei.
A-Line had also completed a design and build of Northland District Health Board's new administration building on West End Ave, which will be Whangarei's first four green star rated building, and the Jim Carney Cancer Centre at Whangarei Hospital.
The company is based in Whangarei, with offices in Kerikeri and Auckland.
The contract process sparked controversy in Kaikohe when the Education Ministry excluded local firm Henwood Builders from the bidding, despite its previous experience at Northland College and its location directly across Mangakahia Rd.
Mr Rintoul said Henwood Builders could end up being subcontracted for some of the work.
The near-total rebuild of Northland College follows controversy over the state of classrooms described at the time as "the worst in New Zealand".
