Thousands of university students have skipped classes to call for urgent action on climate change.

The National Union of Students (NUS) led mass walkouts on Friday, with marches planned in every capital city to call attention to “the biggest issue of our generation”.

“A lot of university students have already been standing in solidarity with the school climate strikers,” NUS President Desiree Cai told SBS reporters.

“The climate crisis is the defining political issue of our generation and an existential threat to humanity.”

Those participating in the walkouts have four key demands for the Federal Government: no new coal mines or CSG operations, stopping the Adani mine, 100 per cent renewable energy by 2030, and a guarantee to help fossil fuel workers to transition to green industries.

“Climate change is going to affect the way that we work in the future, the way that we live, our quality of life ... it's really scary and daunting,” Ms Cai said.

“We need to educate workers to go into renewable industries and use our researchers in order to find new solutions to stop the climate catastrophe.”

Students worldwide are also preparing to gather for global climate strikes on 20 September.

The National Tertiary Education Union called on universities “not to penalise students who decide to take action”, and urged its members to attend the rallies too.

“NTEU members strongly support the stand that students are taking,” union president Alison Barnes said.

“The Federal Government is still treating climate change as an inconvenience, rather than the most important issue facing humanity today.

“Given the current funding freeze that universities are subjected to, and that since 2011 public investment in higher education has been slashed by more than $10 million, we would welcome any increased funding across our sector.

“If Australia is to respond to climate change, we need the research telling us how we might do that.”