They argue this half a billion dollar "lavish gift" was on top of the $4.6 billion already promised to non-government schools by the Federal Government.

Both groups urged the Berejiklian government to fund "chronically under-invested" public schools first.

"Why are our governments bending over backwards for non-government schools, many of which are already overfunded? It's time our elected representatives stop playing political games and prioritise public schools, where the funding is actually needed," P&C Federation president Susie Boyd said.

The NSWTF pointed to figures showing public schools enrolments were growing and were projected to continue to grow strongly over the coming decade while the rate of growth for private school enrolments had halved since 2013.

My School data reveals that in 2016, the latest year for which statistics are available, capital expenditure in the Independent sector was 4.5 times greater per student than in public schools.

In Catholic schools, capital spending was just over twice the rate per student of public schools.

"Infrastructure NSW has revealed that current funding will deliver only a quarter of the learning spaces needed and urged the NSW government to commit to fully funding its School Assets Strategic Plan," NSWTF president Maurie Mulheron said.

"We know that by 2031, our public school system will be accommodating an extra 168,000 students. All of that adds up to an extra 7200 classrooms required by the public system in NSW by 2031 on the State Government’s own predictions.

"This additional half a billion dollars to private schools comes just months after the NSW government signed a funding agreement with the Commonwealth acknowledging that Catholic and Independent schools are currently above their legislated funding entitlement."

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NSW Education Minister Rob Stokes said the increased investment will be delivered through grants under the Building Grants Assistance Scheme, which directs funding towards areas of greatest need.

He added NSW was also the only state to provide an extra $712 million to public schools to complement the Commonwealth’s $1.2 billion Choice and Affordability Fund.

But Ms Boyd said the Federal and State governments combined were already overfunding non-government schools by a "whopping" $160 million.

"There is no reason at all for this new lavish gift of half a billion dollars," she said.

"The state is facing enormous enrolment numbers over the next decade.

"The large majority of these increased student numbers will be in the government school sector, so that is where the majority of funding should go.

"Governments have a direct responsibility for public assets and should not put private assets ahead of that responsibility."