Private schools are outspending Victorian public schools by four to one, splurging on rowing tanks, pilates studios and sky decks.

Some top private schools have spent up to $70 million on capital projects over the past few years as part of a facilities "arms race" to lure students.

The state's biggest spender, Carey Baptist Grammar School, shelled out about $11.4 million in 2014 on a new "learning and innovation" centre at its Kew campus.

The $23 million building, designed by top architects Hayball, features an "audiovisual Imaginarium" with 3D technology and a "United Nations Room" with a sky deck with sweeping views the city.