The federal government has signalled that any cuts to funding for the states that signed on to Gonski would only be borne by public schools, according to NSW Education Minister Adrian Piccoli.
Mr Piccoli and his state and territory counterparts have slammed Federal Education Minister Christopher Pyne over the "bombshell revelation" following a tense meeting in Sydney.
They say they now have even less certainty than they had before the meeting.
"The Commonwealth has implied that, if there is a reduction in funding for the states that have signed up, that indeed that reduction may well only come out of public schools," Mr Piccoli said. "That is of enormous concern to all jurisdictions. I sought, as chair, some clarification about that but none was forthcoming. So that adds a different level of complexity and uncertainty to what is already a difficult and uncertain proposition from the Commonwealth."
Mr Piccoli said the Abbott government's backflip on Gonski was "a matter of trust", not just between governments but for parents across Australia.
"The government made a commitment that there would be no broken promises under the government that they lead and unfortunately that has not come to pass," he said.
He said it was "unacceptable" and "quite an incredible outcome" if reduced funding for schools after next year was only to come out of public schools.
"It's been made very clear to me by the independent and Catholic sectors in NSW that they agree to the split in funds that we've signed up to," he said.
Tasmanian Education Minister Nick McKim said it was "a bombshell revelation that will rock the public education system in Australia to its core".
"We had peace and [Mr Pyne] has lobbed a stick of dynamite into what was a very tranquil pool – and now we are back at risk of ongoing uncertainty and ongoing division between government and non-government sectors in this country."
Mr Pyne denied he had broken an election promise.
"Everything I have said about school funding, every commitment that I have made, we are keeping," he said.
"All the rhetoric and all the posturing from various people in the education sector about money being taken away and six-year agreements, quite frankly, it's all jumping the gun."
The president of the Australian Education Union, Angelo Gavrielatos, said he was disturbed by news that any reduction in funding would be borne by public schools.
"This represents a serious and unprecedented assault on every single public education community across Australia," he said. "Public schools do the heavy lifting. They educate over 80 per cent of students from the poorest backgrounds, over 80 per cent of students with disabilities including the most severe disabilities, over 85 per cent of indigenous students.
"Yet it is public schools that we learn today would suffer the consequences of any cuts in funding as a result of this act of betrayal by Tony Abbott and his government and this breach of contract with the parents, students and teachers of Australia," Mr Gavrielatos said.
The president of the NSW Secondary Principals' Council, Lila Mularczyk, said the assurances schools had around funding were now "in total disarray".
"Disappointment is an understatement," she said. "This is appalling that our students, under the Coalition's alleged student first policy, are now being put back ... this is dreadfully disappointing."
Rachael Sowden, of the NSW Federation of Parents and Citizens Associations, said she was "horrified" to learn that public schools were going to wear the brunt of any cuts.
"We went to the election with a unity ticket and we believed that our children would be better off under the Gonski model, and we are absolutely horrified at what Minister Piccoli came out and said – that public schools are where the funding is going to be reduced."
Mr Piccoli said he expected education funding would be "the subject of hot discussion" at the next Council of Australian Governments (COAG) meeting in December.