NSW's auditor-general says increasing class sizes is one way of coping with a surging student population, but the premier says it's not an option the government will consider.
Acting auditor-general Ian Goodwin has released a report which backs the education department's reforms to overcome under-investment in school but highlights the need for greater cost-cutting measures.
The Planning for School Infrastructure report said further significant savings were only likely "if the department changes some operational policies" and noted "class size is a key determinant of the number of classrooms needed".
But Premier Gladys Berejiklian says the government is "not about increasing class sizes".
"What we are increasing is the maintenance to our schools (and) the number of teachers," she said during Thursday's question time in parliament.
Education Minister Rob Stokes also told reporters "one thing we won't be looking at is any changes to class sizes".
The auditor-general's report notes the NSW school population is expected to grow to nearly 1.5 million students over the next 15 years with more than 80 per cent of those pupils in Sydney.
It reviewed the department's School Assets Strategic Plan which acknowledges under-funding in the state's schools while forecasting they'll need to cater for another 164,000 students by 2031 with 7200 new classrooms.
Opposition Leader Luke Foley opposes any increase to class sizes.
"Last week I said they're softening us up for increased class sizes and I think this report today confirms that," he told reporters on Thursday.
"Increased class sizes are put forward as perhaps the only option in the circumstances to solve the problem, but it's not the only option."
Mr Goodwin's report states 180 schools are operating beyond their permanent classroom capacity.
Another of his suggestions is to convert single-sex schools into co-educational schools and transform selective schools into comprehensive ones.
There are currently 242 empty classrooms in 19 boys-only high schools, 155 empty in 24 girls high schools, 164 empty in 21 selective high schools and 153 empty in 15 specialist schools.
Mr Stokes said the government would consider converting some into comprehensive schools.
"It won't be right for every school, but certainly in some cases there may be a case based on local circumstance to look at new ways to have access to local comprehensive high schools," he said.
The report comes a day after NSW said the Turnbull government's proposed new funding model - dubbed Gonski 2.0 - would leave the state's schools worse off by "millions and millions" of dollars over the next two years.