Documents obtained under freedom-of-information laws and released at a budget estimates meeting on Monday show many of the state's schools are already full-to-bursting, just as the department braces for a major surge in enrolments in the coming decade.

Comparing the number of teachers to classrooms, a measure used by the department to measure a school's capacity, more than 800 public schools across NSW are operating at 100 per cent capacity or more.

That represents 37 per cent of the state's schools. Some 180 schools, or more than 8 per cent, are stretched beyond their limits, the department's figures show.

This includes several schools in inner-Sydney such as Orange Grove Public School, where teachers outnumber spaces by more than 60 per cent. At Bondi Public, it is 40 per cent.

Shadow Education Minister and former school principal Jihad Dib said teachers in high schools were having to use spaces such as metalwork shop rooms to teach English.

"It's a huge amount of pressure on schools and teachers are being forced to find spaces outside of classrooms," Mr Dib said.

He said that students at over-capacity schools in regional areas were most affected by overcrowding and had to travel longer distances to alternative schools.

But in a budget estimates hearing on Monday the Education Minister, Adrian Piccoli, denied that schoolchildren were crammed like "battery hens".

"We have an announced in this year's budget an investment of an additional billion dollars [in the school system]," Mr Piccoli said.

Fairfax Media recently revealed that NSW schools are facing a widening budgetary crisis driven by overcrowding. On its own estimates, the department needs an additional $11 billion in funding by 2031.

The projected boom in school-age public school pupils of 15,000 a year is three times higher than the department's previous assumption of 5000 a year.

In total, the NSW school system will be required to cope with an extra 225,000 students by 2031, 165,000 of whom will be in the public system. About 90 per cent of the increase will be in Sydney.

But Mr Piccoli said the government was taking measures to address the issues in the report and that "several things had happened since then, including the state budget".

But he declined to say how much extra money would need to be found in coming state budgets to meet "huge growth in enrollments".

Mr Piccoli said overcrowded schools used demountable classrooms, stopped taking out-of-area and international enrolments and referred students to nearby schools.

He said that when the coalition government came to power, enrolments had flatlined but was now facing "unprecedented" growth.

Former ABC managing director Mark Scott takes over as secretary of the state's education department on Thursday.

Principals at schools operating at more than 100 per cent capacity had generally decided to keep students within existing buildings rather than building new demountables, the department said.

Mr Dib said the Opposition had sought figures that compared the number of pupils, not teachers, to classrooms and suspected they would show greater levels overcrowding.