Anti-coal mining group Front Line Action on Coal sent press releases to major media outlets claiming the ANZ Bank had pulled its $1.2 billion loan to the miner.
The release came from a fake ANZ email address, contained the bank's letterhead and the names of actual ANZ corporate staff.
It said ANZ had withdrawn the $1.2 billion loan facility it was underwriting to fund Whitehaven's flagship NSW Maules Creek project, only a fortnight after the loan was announced.
When media outlets began reporting the release shortly after midday on Monday, investors started dumping the stock causing it to plummet by 27 cents, or nearly eight per cent, to $3.22.
ANZ was the first to react, alerting media outlets that the press release was false and the loan still existed.
Whitehaven stopped trading in its shares shortly before 1300 AEDT for about 10 minutes to confirm the stunt, and its shares recovered all of the losses to be one cent up at $3.53 by 1500 AEDT.
Front Line Action on Coal accuses Whitehaven of planning to destroy 1360 hectares of koala habitat and forcing farmers off their land through soil damage from the Maules Creek project.
Spokesman Jonathon Moylan, whose phone number was placed at the bottom of the fake release under actual bank employee Toby Kent's name, answered the phone as Toby before admitting to the hoax and apologising to anyone embarrassed or deceived.
He called the hoax a "Yes Men-style stunt", referring to the US activists who have successfully posed as corporate entities, including appearing on the BBC as a Dow Chemical spokesman.
"However what's at stake here is a lot more valuable than money," he told AAP.
In the hoax email, the group falsely suggested the bank would not be investing in coal projects.
Mr Moylan would not comment on the possibility that he or other activists could face criminal or civil charges, jail time or fines relating to releasing false or misleading statements under the Corporations Act.
The Australian Securities and Investments Commission said it was investigating the matter.
Coal mining and coal seam gas drilling are controversial issues in northern NSW, where many local residents oppose activities they say will damage the natural environment.
Protesters tried to block access to Whitehaven's annual general meeting in November until police threatened to arrest them.
If Whitehaven gets federal approval for Maules Creek, it could come online in mid-2013, transforming the company and more than doubling its annual coal production.
However the destabilising presence of embattled entrepreneur Nathan Tinkler's 20 per cent stake on the share register has contributed to a 40 per cent falls in value since April.
Whitehaven managing director Tony Haggarty said the company will co-operate with the Australian Securities and Investments Commission investigation.
"Whitehaven treats matters such as the hoax announcement by Mr Moylan very seriously," he said.
"The integrity of the information provided to the public regarding Whitehaven goes to the heart of the markets integrity and directly effects the price of Whitehaven shares, as well as our many shareholders.
"Whitehaven is also investigating any legal action that may be available to it and its shareholders."