New figures from the Department of Education show the number of demountables used as classrooms spiked to 5093 in April last year, the NSW Teachers Federation said on Saturday.
"The number of demountable classrooms is 30 per cent higher than when the Coalition won office in 2011," said the federation's president Angelos Gavrielatos.
"They are increasingly being used as permanent solutions to the shortage of permanent classrooms which is totally unacceptable".
"And our children are the ones who are paying the price".
The electorates with the highest numbers of demountables were all in Sydney: Riverstone (235), Premier Dominic Perrottet's seat of Epping (178), Castle Hill (136) and Kellyville (134),
Labor education spokesperson Prue Car said the jump from around 4000 demountables in 2011 shows the Liberals-Nationals Coalition "have comprehensively failed to deliver".
The opposition has promised to build several schools in areas with a fast-growing population, mostly in Western Sydney, if voted into government next week.
It also announced on Saturday nearly $18 million to incentivise students to learn a community language on weekends.
After English, Mandarin Chinese is the most commonly spoken language in the state followed by Arabic, Cantonese, Vietnamese, Greek and Italian.
A $100 rebate is on offer for parents whose children pass their end-of-year exams and achieve an 85 per cent attendance record throughout the year in the language classes.
"Speaking more than one language is an increasingly valuable skill in a globalised world. Labor's policy will make it easier and more affordable for every child who wants to learn a language to do so," said Labor leader Chris Minns.
There are 35,000 students studying 60 different languages in more than 250 community language schools across NSW.
Labor also committed to protecting workers in the music industry from sexual harassment and those accessing crisis relief services with a total of $6 million over four years.
Meanwhile, Premier Dominic Perrottet will head out to Penrith in outer Sydney with former minister Stuart Ayres in a push to retain the marginal seat.
With one week to go until polls open, early voting centres around the state are preparing for strong demand.
They are opening from Saturday through to 24 March except for Sunday 19 March.
Recent elections held in Australia, at the federal, state and local government levels, have seen a marked increase in the number of voters turning out in person to vote early, the NSW Electoral Commission said.
"If you will not be able to vote on election day and are eligible to vote early, find your local early voting centre today," commissioner John Schmidt said.
"One of the best ways to plan your vote is by using the Find my electorate tool on the NSW Electoral Commission website".
