He said "tonnes and tonnes of steel" was being used in the latest phase of work to construct invisible strengthening frames stretching from floor to ceiling within the walls.
Other original features of the 1920s building, including ornate plaster ceilings and exposed native timber beams and woodwork, would be returned to their former glory and exterior chimney stacks removed in the 1960s would be recreated in plywood to complete the feel and style of the original structure.
"The work will be done in stages and will be a real balancing act to make sure the girls have somewhere to live and sleep and take meals," he said.
"The cost of building new was horrendous and the strengthening work is just such a contrast to other buildings in the town that are being torn down as earthquake risks.
"There have been misgivings at the costs of strengthening and restoring but in a few years' time people will be saying 'Thank goodness it was done, and done properly'."
The foundation stone was laid in late 1920 for the building, which early boarders and students had called the Old House. Trustees had wanted construction completed then but bricks were difficult to procure and the economic slump of the early 1920s had hit home.
Funds and building progress slowed to a crawl and the ground floor was roofed over and used as a day school for some months before the school officially opened in September 1921.