Former NSW education minister Adrian Piccoli has called for a commission of inquiry into the way Catholic schools spend public money, saying as a practising Catholic he is "deeply disappointed" with their lack of transparency.

Mr Piccoli, who was replaced by Rob Stokes in a cabinet reshuffle in January, said even when he was responsible for approving $500 million in annual funding for Catholic schools in NSW, he had no idea how the money was spent.

Former NSW education minister Adrian Piccoli says there should be more transparency in Catholic schools funding. Credit:Louie Douvis

He said the NSW Catholic Education Commission submitted a signed one-page form which unlocked $500 million.

"There is no transparency back to government about how Catholic schools spend half a billion dollars of taxpayers' money, there is just a one-page form and we have no idea how that money is then distributed," Mr Piccoli said.