Thousands of Victorian teachers, principals and support staff will walk off the job today over a long-running dispute over pay and conditions.

Most public schools in the state will be affected and around 170 are expected to close.

It is the third strike in an 18-month dispute between the Education Union and the State Government over a new workplace agreement.

Union state president Meredith Peace says teachers had no choice.

"We regret any inconvenience and we know our action does cause some disruption," she said.

"We're left with no choice, we're dealing with a government that refuses to negotiate."

Education Minister Martin Dixon says around 65 per cent of public school staff are taking part.

He says today's stoppage is disappointing.

"It's going to cause great disruption to families and great disruption to Victorian businesses," he said.

"There are productive talks going on, talks in good faith, we want those talks to proceed."

Teachers, principals and support staff will rally at Melbourne Park before marching to State Parliament.

There will also be smaller gatherings in country areas.

The State Government says casual relief teachers have been asked to supervise students who show up to school.

The union met with the State Government on Tuesday and expects to meet again next week.

The Government is offering a 2.5 per cent pay rise each year for three years while the union is seeking 12.5 per cent over three years.

Teachers are refusing to participate in out of hours school activities including sport and school camps.

Last week the Government took the union to court to try and prevent the strike but the court ruled in the teachers' favour.