As part of the grassroots campaign, the Catholic Education Commission of Victoria has given principals a document of "talking points" to help them discuss the impact of the changes with politicians.

It says the Catholic school system is "under attack", and provides principals with background information and a list of questions to ask their local MP.

These include: "Do you think fee increases for middle-income families, equalling thousands of dollars, are fair?"

Principals Association of Victorian Catholic Secondary Schools president Mark Sheehan, who is also the principal of Catholic Regional College Melton, said there was a considerable amount of anxiety among parents.

He recently wrote to parents and told them not to expect any massive fee hikes next year, but said there was "understandable anxiety" about Catholic school funding in year 3 to 10 of the new funding model.

"Beyond next year, I am not sure at this stage how the funding of Catholic education will go," he wrote.

Federal education Minister Simon Birmingham called on the sector to abandon its " baseless scare campaign". He said there was no reason for fees at Victorian Catholic schools to rise and that funding would increase by $1.09 billion over the next decade.

"People should be embarrassed to seek special treatment at the expense of other schools when we are seeking to deliver the Gonski model of fairly distributing our record and growing funding," he said. "We're focused on ensuring schools in Victoria and across the country are treated fairly and equitably."

Mr Birmingham has written to every principal to inform them of the changes and has also briefed local MPs on individual school funding.

St Patrick's College, Ballarat principal John Crowley said he did not expect that the proposed changes would lead to significant fee increases at his school.

He said he needed more clarity before communicating with parents.

"The information from Catholic Education Commission of Victoria is at odds with information from Education Minister Simon Birmingham. There is a stand-off of opinions," he said.

"The information we are sending out needs to be accurate."

Michael Gray, the principal of St Joseph's Primary School in Warrnambool, told parents in a letter that "unreasonable fee increases are only a possibility at this stage - not a certainty".

Mr Gray, who is also president of the Victorian Association of Catholic Primary School Principals, visited his local MP, Dan Teehan, to raise concerns.

Catholic Education Commission of Victoria executive director Stephen Elder said he was trying to educate 140,000 families with children in Victorian Catholic schools about the potential impacts of the government's proposal.

"We hope the government will realise the merits of our case and amend its proposals or we will find sufficient support elsewhere in the parliament to spare our parents fee increases," he said.

Education Department data released earlier this week revealed that Catholic education authorities were short-changing needy Catholic schools by up to $1.5 million a year to ensure that fees remained low at their more affluent schools.

This week the federal government introduced legislation into federal parliament for its new funding model, which it argues is much fairer than previous arrangements.