Teachers' Federation Country Organiser Mercurius Goldstein said the union wasn't even aware of the sudden - and illegal - industrial action until after it was over.

"Every faculty has missed out on a teacher for a period of time during the year and that they're still chasing vacancies, which the department has failed to employ qualified teachers for," he said.

"The department has neglected its responsibilities to provide qualified teachers across the whole of rural NSW. This problem is not isolated to Gunnedah. It is being observed in towns across the whole of rural NSW."

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A spokesperson for the Department of Education said members of the union held a "15-minute stop-work meeting from 9am".

"Students were supervised by other staff during the meeting," he said.

The school has "five permanent vacancies", the spokesperson said, and the department is working with the school to fill them "as quickly as possible".

"Three classes have been temporarily merged, with additional teachers added to each class, maintaining the required student to teacher ratios."

Over 30 teachers walked off the job at the school, according to a photo of the incident shared with the Leader by the union.

The Teachers' Federation said in a statement on Tuesday afternoon that the school has a "staffing crisis".

NSW Teachers Federation Deputy President Henry Rajendra said the school has had unfilled teaching positions in English, maths, science, personal development, health and physical education, industrial arts and special education so far in 2021.

"The inability to fill vacant positions and cover for colleagues on leave at Gunnedah High has forced the school to collapse classes or provide minimal supervision when a teacher was not available," Mr Rajendra said.

"Teachers at Gunnedah have taken this stopwork action to highlight the failure of the NSW Government to appropriately staff their school and the impact the continuing staffing crisis is having on student learning."

Mr Rajendra said staff remained concerned about the Education Department's inadequate provision of teachers for public schools across NSW.

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"The staffing crisis is having negative consequences for curriculum delivery to students and placing enormous stress on staff," he said.

"It has been nearly 20 years since the Education Department conducted a comprehensive workforce analysis and the planning necessary to deal with issues of teacher supply and demand."

A spokesperson for the Department of Education said teacher vacancies are down compared to the same time last year.

"The number of teacher vacancies in NSW public schools is less than 2 per cent of overall teaching roles. Given we have 74,000 teachers, this is a very low vacancy rate for an organisation our size," he said.

"The Department is developing a 10 year staffing strategy to address the issues that create vacancies such as sudden enrolment changes, professional development, illness, economic changes and graduate availability."

In February, Barwon MP Roy Butler called for an inquiry into the issue after teachers walked out of the classroom at the Walgett Community College - High School.

Half of the positions at the Walgett school were unfilled in February, the union complained.

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