Greens leader Christine Milne has backed an anti-coal activist whose actions temporarily wiped more than $300 million from the value of Whitehaven Coal, arguing it was part of a "long and proud history" of civil disobedience.

Jonathan Moylan issued a fake ANZ media release on Monday claiming the bank had pulled a $1.2 billion loan from Whitehaven's Maules Creek project on environmental grounds, prompting a share-market sell-off.

Mr Moylan, from Frontline Action On Coal, says the Australian Securities and Investments Commission (ASIC) has seized his computer and phone as part of an investigation.

ASIC has told ABC's 7.30 it is taking the matter very seriously and that a conviction could result in significant jail time.

Senator Milne says she is "really surprised" investors were duped by the fake press release because ANZ has been "quite happily" investing in coal for a long time.

And she has backed the actions of Mr Moylan, saying "the world's facing a climate emergency".

"Yes, there are investors in Whitehaven Coal who may be affected, but everyone is affected by accelerated global warming," Senator Milne told ABC News.

"Campaigners have been pointing out for some time that those investing in coal mines, those driving the expansion of coal exports, are actually accelerating global warming.

"So what we had is an activist... [who] came out and took a direct action in this regard. It's a non-violent direct action.

"There's a long and proud history in Australia of civil disobedience."

Greens senator Lee Rhiannon congratulated Mr Moylan on Twitter on Monday for "exposing ANZ investment in coal mines".

The Coalition has attacked the Greens for their "disrespect" of the rule of law, saying their support for Mr Moylan's actions reveal the party's "extreme political tendencies".

"The Greens self-righteously presume that they have a monopoly on political, social and environmental rectitude and that therefore the end always justifies the means," Liberal senator Eric Abetz said.

"This is the epitome of extremism."

Acting Opposition Leader Warren Truss says the prank cost the savings of Australian families, and has called on Labor to reassess its relationship with the Greens because of their support of the action.

Parliamentary Secretary to the Treasurer, Bernie Ripoll, says it is "irresponsible" for anyone to endorse the activist's behaviour, describing it as "potentially illegal".

"There are legitimate ways to protest and encouraging this kind of extremism, that hurts shareholders and damages investment, shouldn't be one of them," he said in a statement.

"It is appropriate that ASIC is investigating this incident fully as to whether there has been a breach of the Corporations Act.

"Let's leave it to the authorities not the politicians."

ASIC speaks

Senior Labor MP Joel Fitzgibbon, whose electorate covers the NSW Hunter Valley, wants ASIC to make an example of Mr Moylan so it will act as a deterrent to other activists.

ASIC commissioner Greg Tanzer told 7.30 the country depended on the integrity of the stock exchange.

He would not go into details of ASIC's investigation, but says the body will move swiftly if there is potential criminal charges.

"The penalties that apply here really are quite severe, and they reflect the importance of market integrity to the Australian economy as a whole," he said.

"So if you're convicted of an offence of disseminating false or misleading information to the market that could impact on market securities, [you face] fines of up to nearly $500,000 and a potential term of imprisonment of up to 10 years."

Whitehaven Coal has recovered the losses but says it has had complaints from many shareholders who sold when the price was down and lost money.

The company is exploring its legal options against Mr Moylan and also says it will complain to the Press Council about media reporting of the fake press release.

Whitehaven's chairman, former deputy prime minister Mark Vaile, is furious about the deception as well as the Greens' support for Mr Moylan.

"To have someone who has the respect of the electorate across Australia in high political office be prepared to go out and support fraudulent and arguably illegal activity like this, beggars belief," he said.

'Climate emergency'

Senator Milne, meanwhile, says the recent extreme heatwave and bushfire emergency is an example of what climate scientists have been warning of, although she has stopped short of making a direct link to climate change.

"You can't say that any particular hot day, any particular storm, any particular fire is a direct climate change fire or flood or storm or hot day or whatever.

"What you can say though is that the scientists have demonstrated absolutely that... the increased intensity (of these events) is being driven by global warming.

"We have just had the hottest days ever in Australian history and the climate records are showing that."

Mr Truss says it is being "utterly simplistic" to suggest the fires are a result of climate change, arguing that Australia's climate is "changeable".

"These comments tend to be made on hot days rather that cold days," Mr Truss told reporters in Brisbane.

"We have hot times, we have cold times. We have periods when it's raining, we have periods when it's dry."

On Monday, Prime Minister Julia Gillard said it was not appropriate to blame climate change on any one particular weather event, but added "we do know over time that as a result of climate change we are going to see more extreme weather events and conditions".