A survey of public schools found more than 9800 lessons were without a teacher each day, with 87 per cent of schools impacted.
Those worst affected were rural schools, those in Sydney's south and west and schools that operated for a specific purpose.
In high schools, students in nearly 30 per cent of uncovered classes were left to their own devices with minimal supervision, the state government said on Tuesday.
The teachers' union said the data came "as no surprise".
"The teacher shortage in NSW public schools is a direct consequence of the former government's wage cap that artificially suppressed teachers' pay," NSW Teachers Federation acting president Henry Rajendra said.
"The wage cap made the profession less attractive."
The previous government capped wages except where productivity gains could be realised, holding annual wage growth at 2.5 per cent by the end of its 12-year term.
The new Labor government recently struck a deal that made NSW teachers at the bottom and top of the pay-scale the best paid in the country.
The deal also lifted casual teacher pay rates.
But Mr Rajendra said more work was needed to address "unmanageable and unsustainable workloads" to make the profession attractive again.
Education Minister Prue Car also laid blame at the foot of her predecessor and said work was ongoing to attract and retain more teachers.
"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," she said.
The data was released ahead of Ms Car's scheduled grilling by upper house MPs at the annual budget estimates hearings.
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