The NSW Education Department has been planning for a major expansion in the use of demountable classrooms in the coming decade as the state opposition says internal forecasts show it has plans to "double" their use to meet surging enrolments.

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.

Families and students at Eastwood Heights Public School, who are worried about the increasing number of demountables. Credit:Christopher Pearce

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.