An amendment to the Anti-Discrimination Act, to be debated by Parliament later this month, could allow religious-based schools to give admission preference to students who share their religious beliefs.

Education Minister Jeremy Rockliff says the changes will provide consistency and certainty to Tasmania's faith-based schools.

"The Anti-Discrimination Amendment Bill will bring Tasmanian law in line with all other states and territories," Mr Rockliff said.

But commissioner Robin Banks has flagged serious concerns with the bill, which she has vowed to take up with parliamentarians.

"The proposal is inconsistent with the purpose of discrimination laws and with internationally recognised human rights," Ms Banks said.

"Rather than protecting religious freedom, the proposed approach could undermine the parents' choice by making it the school's choice that prevails."

Ms Banks argued the changes would not provide the protection for religious schools that its proponents were seeking.

She said schools could already ask permission to give preference to a child of the school's religion if there was competition for limited places.

Ms Banks said the changes would only complicate the process, tying parties up in tribunals with lengthy legal and religious argument.

Ms Banks rejected claims existing measures were draconian or onerous.

Gay rights activist Rodney Croome has slammed the legislation as a "bigots' charter", fearing gay or transgender students and parents could be also refused enrolments.

But Mr Rockliff has sought to appease those concerns.

"The bill specifically prohibits enrolment discrimination on any other grounds other than the student's religious beliefs, affiliations and activity," he said.

Greens justice spokesman Nick McKim again urged Mr Rockliff to withdraw the legislation.