Nearly 10,000 lessons in NSW are left without a designated teacher every day due to a shortage of casual teachers, a government survey found.
NSW Teachers Federation Acting President Henry Rajendra from Wollongong is not surprised.
"The teacher shortage in NSW public schools is a direct consequence of the former government's wage cap that artificially suppressed teachers' pay," Mr Rajendra said.
The NSW Department of Education survey published on October 24 found there is a daily shortfall of more than 3000 teachers across the state.
Regional schools in southern NSW which includes Illawarra have an estimated casual teacher shortfall of 42 per cent.
The Regional South School Performance Directorate area also covers Goulburn, Camden, Campbelltown and Yass.
As there are not enough casual teachers to substitute every lesson, the survey found many students are left in alternative supervision arrangements, lessons with minimal supervision or the lessons are cancelled.
The NSW Teachers Federation said the recent wage agreement, which takes effect this week, will help replenish the teacher numbers.
"By paying teachers what they are worth, expanding the pool of teachers available, whether they are casual, permanent, at the early stages of their career or highly experienced, we can begin to reverse the damage done," Mr Rajendra said.
The NSW starting teacher salary will increase from $75,791 to $85,000 and the top teacher salary will increase from $113,042 to $122,100.
Mr Rajendra said more work is needed to address the "unmanageable and unsustainable workloads" to attract more people to the job.
The state survey found 28 per cent of high school lessons without a designated teacher was covered by minimal supervision while 40 per cent of primary school lessons needing supervision were covered by merging classes together.
The teacher shortfall has also resulted in the cancellation of library lessons, additional support programs, and professional development.
NSW Deputy Premier and Minister for Education and Early Learning Prue Car said the survey shows the extent of the teacher shortage crisis.
"It is vital for a child's education that they have a qualified teacher in front of them for every lesson, and that is what we're working towards," Ms Car said.
Sign up for our newsletter to stay up to date.