This is a breakdown of the finances from a public high school in western Sydney which would be called "a bad school" if you had to send your kid there. The "per student" column is what you care about: http://i.imgur.com/3teWTIi.png
Same thing for a very elite private school in a nearby part of Sydney. Again, pay attention to the per student part. http://i.imgur.com/2Y5KqHF.png
Gonski was about addressing the massive difference in what the federal government gives to the different sectors; to shift them away from being subsidisers of private education and into a position of being supporters of education. It was primarily a huge injection of federal funding into the public sector and a very small adjustment to what's happening in the private sector. Ideally they would've cut money from private, but they can't because the Australian public are idiots who think that they'll be rich and connected enough to get a kid into Riverview or Kings one day. The only outlay from the government that wasn't slated for public school investment was going to small independent schools like the little one attached to your local church.
There isn't really anywhere for Pyne to take the money from except for public because nothing of significance was promised outside of those two sectors. He's a piece of shit for doing it, but every education minister already knew that was the case before he said it. This is also why Gonski and public education are spoken about as if they're the same thing. It was a last ditch effort to halt the flight of students out of the public system into independent schools by attempting to achieve some sort of funding parity.
Yes it's an attack on public education, but we already knew that because it's LNP education policy. The ministers are only shitty because it means they can't siphon money out of their education budgets and cover it up with federal cash.