Thousands of Australians have marched in support of science and evidence-based policy as demonstrations kick off around the world on Earth Day.

People of all ages carried homemade banners, including "we need thinkers, not deniers" and "science, not silence".

Hundreds of thousands are expected to demonstrate in the global March for Science, which will be centred on Washington DC in response to US President Donald Trump's proposed budget cuts to science.

In Sydney, former Liberal leader John Hewson told a crowd of about 3000 people that it should be a global embarrassment that Australian politics had become so opportunistic that politicians seemed no longer interested in the substance of the issues.

Dr Hewson said: "Short-term politics is the main impediment to scientific and policy debates we should be having in this country."

He said that former prime minister John Howard had told a conference in London in 2013 that he'd "ended up an agnostic when it comes to climate science", preferring instead to rely on his instincts.

"I worked for the guy for eight years and he spent a few years working for me. I wouldn't trust his instincts, let me tell you," Dr Hewson said.

In an apparent reference to Prime Minister Malcolm Turnbull's clean coal and Snowy Hydro announcements, Dr Hewson said there was no energy transition policy to deal with climate change.

"Every day I wake up and hear a new initiative: one day it's going to be hydro, the next day it's going to be coal.

"There is no substantive response to the most significant moral, economic and social challenge of this century."

Other speakers in Sydney included UNSW climate scientist Angela Maharaj and sociologist Eva Cox.

In Melbourne organisers said 4000 people marched to state parliament. Speakers included former Labor science minister Barry Jones, who said that science advocacy was not a one-shot vaccination, people needed to keep up the pressure.

Professor Jones said that politicians no longer ask "is it true?", they ask "Will it sell?" in development of public policy.

Organisers of the March for Science Australia told Fairfax Media that 10,000 had demonstrated nationally, including in Canberra, Brisbane, Adelaide and Perth.

In Sydney, the NSW Police riot squad was filming protesters at the demonstration. 

NSW Police said the filming was "for evidence gathering" to be used only if the march had not peaceful. A police spokesman said facial recognition software was not used.