Surging population growth in parts of Sydney and a growing preference for public education among parents has left the state's education department facing projected funding shortfalls and an expected growth of more than 175,000 pupils by 2030.

But documents released under freedom-of-information laws show the department's response to that crush includes forecasts that demand for demountable classrooms will far outstrip the 5100 currently on school and TAFE sites.

The projections are contained in a confidential tender circulated last year to private consultants whose advice the government was seeking to examine "all possible cost effective options" to "expand [its] existing demountable fleet" by 2030.

The tender document forecasts the government's total stock of about 6100 demountables would all be fully deployed between next year and 2020, requiring the expansion.

"Planning data identifies significant growth in student enrolment numbers showing a net demand for 5796 demountable teaching spaces in NSW government schools," says the document, obtained by the state's opposition.

Opposition Leader Luke Foley said the department's plans would amount to a "doubling" of demountables, which was "not acceptable" when the government's coffers were overflowing with privatisation and stamp duty revenues and it was embarking on record public works spending.

"They're spending [$16.8 billion on Westconnex] while hundreds of thousands of kids will be learning in demountables," Mr Foley said. "Demountables are far from ideal places for kids to learn. This is a plan to 2031: it's a very long stopgap".

But Education Minister Rob Stokes, while refusing to specify how many demountable classrooms the department was planning on constructing, accused Labor of working from an outdated consultants' report and getting its sums wrong.

"We are spending $4.2 billion on new schools and classrooms over the next four years, which will create 1500 new classrooms," Mr Stokes said. "Enrolments are soaring due to a baby boom, the strong NSW economy attracting young families and world-class academic standards."

Earlier this year Mr Stokes declared demountables "here to stay".

He argued the old style of demountable classroom could be "reconceptualised" as structures designed to permanently house students but which could be moved when required as part of the government's solution to boost school capacity in areas of sustained enrolment growth.

But at Eastwood Heights Public School on Sunday primary school parent Justine Taylor said the temporary structures, though now airconditioned, were less comfortable learning environments and restricted the school grounds.

"As soon as they get out of the classroom [kids] want to run; they've been sitting all day," she said. "There's nowhere to run".

Ms Taylor said the school's enrolments had grown rapidly from the time her daughter Annabel, now in year 2, had started kindergarten, but the department had failed to respond to the signs of local population growth with permanent buildings: "They've been building units and units and units [here and] no family can afford a house."

The state government declined to release the advice it received from its tender for developing a demountable expansion strategy, which was awarded to KPMG consultants, declaring it cabinet-in-confidence.

Currently one in 10 classrooms in NSW are demountables, a third of which have been standing for longer than a decade.

Analysis of departmental figures has shown there are 3.7 demountables per 1000 students in the city's east, compared with 7.4 per 1000 in the north, 7.3 per 1000 in the west and 7.1 per 1000 in the south-west.

Labor MLC Daniel Mookhey accused the government of shrouding its demountable plan in secrecy.