An activist who distributed a fake press release that caused a coal company's share price to plummet is unlikely to be jailed after prosecutors decided not to request a custodial sentence.

Jonathan Moylan appeared in the Supreme Court in Sydney today after pleading guilty to disseminating false or misleading information affecting market participation.

Moylan distributed a fake statement to the media purportedly from the ANZ bank in January 2013, which said the bank was withdrawing $1.2 billion in funding from the Maules Creek mine project, in north-west New South Wales.

The false information was published by some media outlets and caused a temporary $314 million drop in Whitehaven Coal's market value before a trading halt was put in place and the hoax was revealed.

In a signed apology read to the court today, Mr Moylan said: "Those who traded on that day have every right to feel deceived as a result of my actions."

But crown prosecutor David Staehli SC told the court he was not seeking a custodial sentence.

Mr Staehli said Moylan was not seeking a financial advantage, and that made the case unique.

But he said Moylan had researched how Whitehaven Coal's shares responded to good and bad news before issuing the release.

"Mr Moylan knew the information was false. He engaged in careful and deliberate planning over four days," Mr Staehli said.

Moylan's supporters staged a silent vigil outside the Supreme Court in Sydney before the sentencing hearing. ( ABC News: Claire Aird )

Moylan's barrister, Robert Sutherland SC, argued the media, not Moylan, was to blame for the fall in Whitehaven Coal's share price.

Before the hearing, more than 100 of Moylan's supporters massed outside the court, appealing for leniency.

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Many were representing the Maules Creek Community Council protest group, which opposes the mine, and said Moylan was not acting for personal gain, but to protect the environment.

Maules Creek farmer Rick Laird, who is campaigning against the mine, was among Moylan's supporters.

"It's a very stressful time for Jonathan and he just needs that bit of extra support, so we've made the trip up from Maules Creek to to just show him that," Mr Laird said.

"Even though we didn't agree with the action that he took, he did it for us and that's why we support him."

The offence has a maximum penalty of 10 years in jail, and can attract fines of up to $765,000.