A relief teacher at a New South Wales high school, who is also a Greens candidate, is being investigated by the Department of Education over remarks made at a candidates’ forum about the upcoming student climate strike.

Will Douglas, who has worked casually at Moruya high school on the state’s south coast since 2006, has been told he will not be offered any more shifts at the school while the investigation takes place.

It comes as the principal of a Victorian Catholic school warned students that striking was an “unapproved” absence that could lead to a zero if they missed any official tests on the day.

Inspired by Swedish student Greta Thunberg, the movement calls on students to partially strike from school to protest government inaction on fighting climate change. More than 50 rallies, in Australian capital cities and regional towns, are planned for 15 March.

Douglas, who is contesting the seat of Bega in the upcoming state election, appeared at a lunchtime candidates’ forum organised by Youth Action at the Moruya golf club last Thursday, alongside local Liberal MP Andrew Constance and Labor candidate Leanne Atkinson.

Douglas said he attended the forum with the knowledge of his superiors at the school.

During the forum, which was attended by students from a range of high schools, Douglas said “please don’t forget March 15 the climate strike … if there’s something happening at your school will you please get online and register because there’s a whole community out there wanting to support you guys, young people, in that strike”.

The remarks were published in News Corp’s Daily Telegraph on Monday, in an article that also revealed a complaint had been made to the department and that an investigation was under way.

Douglas told Guardian Australia he had no knowledge of any investigation until contacted by the newspaper and he did not know who had made the complaint.

He said the principal told him this week he would not be working any more shifts at the school until the investigation was complete.

“I was speaking on my own time in my lunch break as a Greens candidate at a youth forum,” he said. “Now I don’t have any work. It shouldn’t be this hard to speak up for climate action.”

Douglas said Moruya high was a wonderful school, and that he had been treated with nothing but respect by the principal. However he believed the complaint itself was “politically motivated”.

A spokeswoman for the department said they were making inquiries into the matter but that it would be inappropriate to comment on the employment status of an individual teacher.

“While the department understands students may be passionate about a range of issues, all students who are enrolled at school are expected to attend that school whenever instruction is provided,” she said.

“Staff members providing advice contrary to departmental policies may be subject to allegations of misconduct.”

Meanwhile in Victoria, the principal of Catholic girls school Siena College sent a letter to students and parents last week warning that striking students could receive a zero if they missed official tests and assignments on the day.

“We acknowledge the students’ right to give voice to their very genuine concerns regarding the federal government’s record on climate change,” the principal, Gaynor Robson-Garth wrote.

“Attendance at the strike will however be an ‘unapproved’ absence from school. If a student chooses to participate in the strike, VCE school-assessed coursework and other assessment tasks will be recorded as not assessed (NA) and no alternative opportunity will be provided to complete any missed assessments.

“The decision to participate in the strike is entirely a decision for parents and students. The strike is not endorsed by the college.”

Last November, thousands of Australian students walked out of school to attend rallies across the country. Since then, strikes have been organised in the UK and the United States, and 15 March has been billed as a global day of action, with further strikes organised for the US and Europe on the same day.

In February, the NSW education minister, Rob Stokes, told students that they “should be at school” on the day.

Student Doha Khan from Adelaide told Guardian Australia on Monday the students had three demands: stopping the Adani coalmine, no new fossil fuel projects and 100% renewables by 2030.