The Victorian opposition claims non-government schools stripped of payroll tax exemptions will also have to pay COVID-19 debt and mental health levies.
The Victorian government has been accused of a triple tax on high-fee schools, exposing independent and Catholic education providers to more levies by removing their payroll tax exemption.
About 110Â non-government schools in Victoria are set to be stripped of their longstanding payroll tax exemption from mid-2024 under a state budget measure announced last week.
The move is forecast to net $422.2 million across the next four years as the Andrews government tries to rein in mounting debt following the COVID-19 pandemic.
Victorian businesses with national annual payrolls of more than $10 million are also facing a COVID-19 debt levy from July on top of the state's already implemented mental health levy.
Private schools, hospitals, charities and local councils don't pay the mental health levy under existing payroll tax exemptions.
Paring back the payroll tax exemption would leave some non-government schools with payrolls of more than $10m exposed to both the COVID and mental health levies, opposition education spokesman Matt Bach said.
"This will simply mean that they won't just have to pay the initial schools tax - they will have to pay a triple whammy of taxation," he told reporters at state parliament on Wednesday.
Mr Bach said Commonwealth figures showed 69 Victorian independent schools have a national annual payroll of $10m or more.
That figure does not include Catholic schools.
Education Minister Natalie Hutchins will determine which non-government schools keep their exemption with the consent of the treasurer, the budget papers state.
Asked if Victorian schools would also be liable to pay the mental health levy, Ms Hutchins said she was unaware of any with a national annual payroll of more than $10m.
"But certainly our commitment to mental health is absolutely undeniable," she said during Wednesday's lower house question time.
Opposition finance spokeswoman Jess Wilson said the schools tax had not been thought through.
The proposed tax reforms are expected to pass the Labor-majority lower house on Wednesday evening before being scrutinised in the upper house.
The Victorian government has been contacted for comment.