Fairfax Media has confirmed the donation, paid in two instalments this year, is the second-largest in the ARM's history.

It is understood the first instalment of $50,000 was paid by an entity owned by Mr Packer, while the remaining $200,000 was from the gaming billionaire himself.

The donation is the largest since Prime Minster Malcolm Turnbull - a founding member of the movement and chairman from 1993 to 2000 - stumped up almost $3 million ahead of the Republican referendum in 1999.

Other high-profile supporters include Ambassador to the United States Joe Hockey, Qantas chief executive Alan Joyce, actor Hugh Jackman, businesswoman Janet Holmes à Court, former Queensland premier Anna Bligh and NAB chairman Ken Henry.

The chair of the Australian Republican Movement, Peter FitzSimons, would neither confirm nor deny the donation from Mr Packer on Thursday night, saying only "We are going bloody well, and will make some announcements in due course. But you ain't seen nothing yet.

"This country is coming of age, and the support we are getting from all quarters, rich, poor and the whole mob, has been inspirational."

FitzSimons, a Fairfax Media columnist and author, is campaigning for a plebiscite by 2020 asking if voters want an Australian head of state. A petition he started on Change.org calling for a republic has almost 14,000 signatures.

Earlier this year FitzSimons secured signatures from all but one of the state and territory leaders on a declaration calling for an Australian head of state.

WA Premier Colin Barnett was the only leader not to sign the statement, which said: "We, the undersigned premiers and chief ministers of Australia, believe that Australians should have an Australian as our head of state."

Mr Barnett said although he supported a republic he believed that not enough time had passed since the 1999 referendum.

The Australian Republican Movement has grown in the past 12 months, with a swelling membership and new paid employees.

It is understood the ARM is about to announce a new membership structure and the appointment of a campaign director, who will be charged with laying the groundwork for a plebiscite.

A source said the candidate was an experienced campaigner from the conservative side of politics.

Fairfax Media has attempted to contact James Packer for comment. Mr Packer's late father, Kerry, also supported the republican movement.