Teachers will walk off the job for four hours on Friday, the day before the by-election to replace former chief minister Michael Gunner in the relatively safe Labor seat of Fannie Bay.

The Australian Education Union says teachers want the government to end their pay freeze in the wake of mounting workloads.

Earlier this year they voted to reject an offer that continued the pay freeze until the end of 2024 but provided lump sum payments of $4000, after an agreement was approved, and annual payments of $2000 for the life of the four-year deal.

Branch secretary Adam Lampe said the union would not countenance an agreement that threatened the ability to recruit and retain teachers in the NT, and which seriously undermined the viability of the public education system.

"Members have reported significant challenges in staffing, workload, and resourcing since the start of 2022," he said.

"The AEU will not support an agreement which will make these issues worse."

The industrial action has also won support from the Independent Education Union which represents teachers in non-government schools.

"Teacher shortages, unsustainable workloads and uncompetitive wages are issues for teachers right across the country and the Northern Territory is certainly not an exception," IEU branch secretary Terry Burke said.

Friday's strike is expected to involve schools across Darwin and Palmerston as well some in regional areas including Humpty Doo and Taminmin.

Earlier this week, the NT Department of Education said all schools would remain open.

"Every effort will be made to minimise impacts on learning and ensure appropriate supervision is in place for students to ensure their safety and wellbeing," the department said.

"However alternative arrangements during this time, such as modifying timetables and combining classes may be required."

Education Minister Eva Lawler said an enterprise bargaining agreement was still being negotiated with teachers and the union.

The NT government's four-year public sector pay freeze for more than 20,000 public servants was introduced in November 2020 in a bid to save more than $400 million amid ongoing territory government deficits and climbing debt.

Australian Associated Press