Let’s face it, everyone was caught by surprise when the prime minister, Malcolm Turnbull, announced a new pathway to school funding equity – one that would overturn a two-decade mantra that no school would lose a dollar. Schools that are currently funded ahead of their entitlement will be pared back, others face a more leisurely rate of increase in their public funding – behind schools that can clearly demonstrate greater needs.

The proposed national needs-based funding model will do away with decades of special deals which, among other things, have even seen Catholic schools funded well ahead of independent schools.

The responses have followed a well-worn ritual, with Catholic school authorities crying foul and the ALP adding to the chorus. Not much has changed in half a century. But in a real break with the past, a conservative government is leading the charge to build some sanity in a funding system that lost its way well over a decade ago. They actually didn’t have any alternative – no one can come up with any narrative to justify the inconsistent, opaque and risible way we fund schools.

But in defiance of current realities, the response from Catholic school authorities and supporters comes straight from decades-old talking points. Once again we hear that Catholic schools enrol the needy and direct funding their way, that it is all about choice, that parents make a sacrifice, that fees will go up – and they only get a fraction of the public funding going to government schools. And anyway, if all the students in Catholic schools went to public schools it would cost governments a small fortune.

Let’s deal with the easier ones. In measures of student/family advantage Catholic schools lie below independent schools but well above government schools. Of course they have needy students, but long ago even the Catholic bishops acknowledged government schools enrolled the poorest Catholics. We are told that Catholic schools distribute public funding according to need, but two previous audits and a current report strongly suggest otherwise.

So what about choice? Tony Abbott has come out of the blocks saying this is now at risk. But on average, only one out of every two Australian families might be able to afford a private school for possibly one of their children. True, many parents do make a sacrifice, but the whole notion of choice – Australian style – leaves out half the population.

Then we have fees. Strangely, private school fees go up when public funding doesn’t – but they also go up when public funding does. The only constant is that fees keep going up. There was a time when Australian private schools had to charge fees; their public funding fell well short of the level of resourcing of government schools. But all that has now been turned upside-down. The public funding, both commonwealth and state, is so high that fee income now resembles icing on the government-funded cake.

This is because the vast majority of Catholic schools now receive between 90.5% and 98.5% of the public funding going to government schools with similar needs. They are becoming publicly-funded schools. Many are funded at around 110%. Average funding levels are lower but averages mean very little when it is only government schools that must accept every child – a very expensive and challenging obligation. The mismatch between levels of public funding and such an obligation is a looming problem.

Ah, but here comes the killer argument: if all the students in Catholic schools fronted up to their local public school it would cost the taxpayer dearly. This was the argument mounted when state aid to church schools symbolically began in Goulburn in 1962. The local Catholic schools were going broke and their students were packed off to the local public schools. The spectre of the bigger cost of enrolling all students has spooked governments ever since. Indeed so the argument goes, sending your child to a Catholic or independent school is doing everyone a favour and is your little blow for equity.

The cost-saving argument has always been overstated but it has well and truly passed its use-by date. If the Goulburn scenario was repeated today, it would now cost only 1% more to educate all Goulburn’s Catholic school students in government schools. If economies of scale were factored in it would cost even less and almost certainly governments would end up ahead.

The lesson out of all this is that advocates for schools and sectors need to bring themselves up to date. Catholic schools have had a good run but our education framework has considerably changed and we have the evidence to drive a better informed debate.

Wouldn’t that be nice for a change?

Chris Bonnor and Bernie Shepherd are fellows of the Centre for Policy Development