The gift, which is believed to be one of the largest endowments ever to a school, will fund a 1400-seat underground auditorium as well as a new library and learning centre opening in 2021.

Mr Snow said the private school was a Canberra institution and the new build would keep the heritage of its 90-year-old campus alive.

"Anything I'm associated with, I try and commit to doing as good a job as I possibly can, and I would like to see this school be the high-water mark for independent education in Australia," he said.

Principal Justin Garrick said the gift, which follows Mr Snow's earlier $8 million endowment to Canberra Grammar in 2014, would fast-track both its campus redevelopment and a planned line of Indigenous scholarships.

While the new building will replace the "ailing 1960s frontage" of the school, Dr Garrick said its new glass-walled breezeway also honoured the original design of the campus.

The planned auditorium and music centre, due to open in 2021, underscored the school's strong musical focus, he said, and would create another medium-sized concert venue for Canberra.

Mr Snow owns Canberra Airport and sits among the country's wealthiest people, with an estimated fortune of $1.8 billion.

But before he entered the business world, he and his brothers were students at the Canberra Grammar of the 50s. Back then, he said, "there weren't the opportunities the kids have today". For one thing, the music department was still housed in a wooden hut out on the grounds.

Now the centre will overlook the campus, with a library on the top floor and classrooms and rehearsal rooms sinking down into subterranean.

Dr Garrick said it would more than double the school's music department, which was already struggling to keep pace with the school's 30-plus orchestras, bands and choirs.

Mr Snow praised music as "the pinnacle of the arts" and said targeted philanthropy, such as his donations to Canberra Grammar and to equine industry, had power.

"We've all got our different ideas about how we'd like Australia to surge ahead," he said. "I have enough to live on, and my family have enough to live on.

"This building will change the school forever...But [it's] heritage is maintained.

"To see the quadrangle, bell tower dining room, all very significant buildings very evocative to old boys, but underneath, is a modern powerhouse for the future."

The school did not say how much of Mr Snow's endowment was earmarked for Indigenous and equity scholarships but said it would be a "catalyst" for a major initiative in the years ahead.

Mr Snow and his family have also donated more than $26 million to those in need over the past 28 years through their charity the Snow Foundation.

Construction is due to begin on the new build at Canberra Grammar in the coming months.

Each year, the school receives about $7 million in Commonwealth and ACT government funding, more than $34 million in parent fees and another $2 million from other private sources, according to the latest available figures.

When asked if ACT public schools had ever received large donations, an education directorate spokesman said all schools were funded by the government.

But private schools have benefited from generous alumni on more than a few occasions in Australia - in the 90s, Melbourne Grammar School was also famously gifted $10 million by old boy Jack Morrow.

And in 1950, The Sydney Morning Herald reported one Mrs. Violet Madeline MacAnsh had left a bequest of £400,000 to the King's School in Parramatta

Today, that sum would be worth about $20.7 million. Mrs. MacAnsh lived on an estate just near the ACT border in Harden and her father was one of the private school's very first pupils, the paper reported.

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