Shares in Nathan Tinkler's coal company were placed in a trading halt after a fake media release claimed funding for Whitehaven Coal had been withdrawn.
Whitehaven Coal shares plunged 6 per cent to $3.31, wiping $300 million off the company's value, before trading was stopped just after noon (AEDT).
The fraudulent media release claimed ANZ had withdrawn $1.2 billion from Whitehaven's Maules Creek project in north-east New South Wales.
It said the decision was made because of "volatility in the global coal market, expected cost blow-outs and ANZ's Corporate Responsibility policy".
ANZ has confirmed the media release is a hoax and was not issued by the bank.
The Maules Creek mine, about 18 kilometres north of Boggabri in the Gunnedah Basin, is set to be one of the biggest open-cut coal mines in the world, extracting 12 million tonnes of raw coal a year.
The project was approved by the NSW Planning Assessment Commission in October.
If given the final go-ahead from the Federal Government, Whitehaven expects production to start in the middle of this year, with operations predicted to last around 30 years.
Environmentalists have raised concerns about the critically endangered white box gum woodland in the nearby Leard State Forest.
A group of activists called Frontline Action on Coal, who have camped out at the Maules Creek site for more than 150 days, claimed responsibility for the hoax.
Frontline Action on Coal spokesman Jonathan Moylan said in a statement that the group would continue to campaign against the ANZ bank until it shifted support to "ethical" investments.
"ANZ is putting their customers' money at risk by investing in this marginal and controversial project," he said.
The Australian Securities and Investments Commission (ASIC) is looking at whether there has been a breach of the Corporations Act.
Shares in Whitehaven shares recovered most of their earlier losses to close down 0.5 per cent at $3.50.