* Tinkler pursued by federal tax office
* Sentia Media joins list of creditors
* Tinkler's helicopter and jet seized
By Jane Wardell
SYDNEY, Dec 10 (Reuters) - Australia's tax office is seeking to wind up the holding firm of Nathan Tinkler over unpaid debt, one of the highest profile lawsuits filed against the coal baron, who had his private helicopter and jet seized by liquidators in another recent case.
Tinkler, 36, became Australia's youngest billionaire by riding on the back of the country's mining boom, but a slide in coal prices has hit his net worth.
A series of lawsuits over unpaid bills and commercial disputes have raised questions about the future of Tinkler's main asset, a near one-fifth stake in Whitehaven Coal , Australia's largest independent coal miner.
The Deputy Commissioner of Taxation launched action on Monday in the New South Wales Supreme Court, pursuing Tinkler Group Holdings Administration Pty Ltd over an unspecified debt.
The Australian Tax Office declined to comment on the case, which will return to court in February after the Christmas break, or provide details on the amount of money owed.
Sentia Media, a media monitoring service with corporate and government clients, joined the action as a supporting creditor. It also declined to comment.
Tinkler's spokesman did not immediately respond to an email for comment.
Liquidators have been appointed to three firms of which he is director, horse racing business Patinack Farm Administration Pty Ltd, Mulsanne Resources Ltd and TGHA Aviation Pty Ltd, over debts totalling more than A$34 million ($35.66 million).
A receiver took possession last week of Tinkler's executive jet and his helicopter.
Two further wind-up petitions, against Tinkler's Queen Street Capital and Aston Copper Ltd, are due to be heard in the Queensland Supreme Court on Wednesday.
Whitehaven shares rose 2.7 percent on Monday, but at A$3.08, they have lost 44 percent since a $5 billion merger of Tinkler's companies Aston Resources and Boardwalk Resources with Whitehaven in April.
That's cut Tinkler's holding, which sources have told Reuters is heavily leveraged, to around A$600 million, compared to A$1.1 billion at its peak.
Tinkler's main backer, U.S. hedge fund manager Farallon Capital Management LLC's asset manager Noonday, has been looking at options including pressing for the sale of shares or converting some of the loans into equity.
Tinkler paid creditors A$500,000 last month to stop wind-up petitions against Tinkler Group Holdings Administration, but the federal tax office stepped in instead as a creditor.
Tinkler has had some success in resolving court action.
He avoided a public spat with an Irish racehorse stud owned by Sheikh Mohammed bin Rashid al-Maktoum, the ruler of Dubai, which dropped a separate court action last week.
Kildangan Stud, owned by Sheikh Mohammed's global breeding operation Darley, had subpoened Tinkler over missed payments for breeding fees relating to its stallions and Tinkler's broodmares.
Darley managing director Joe Osborne said the debt, outstanding for a couple of years, was settled on the eve of court action.
Tinkler has also flagged his intention to come up with as much as A$6 million by the end of the year to repay creditors and pull Patinack Farm Administration, Australia's largest horse breeding racing operation, out of liquidation.
Insolvency firm Anthony Matthews & Associates said last week that Tinkler and other directors of the company had come up with funds for immediate wage payments due to the company's 147 staff and said they intended to pay off all debts by the end of the year.
A source with knowledge of Patinack Farm, who was not authorised to speak publicly, said debts at that business are likely to total between A$5 million and A$6 million. ($1 = 0.9533 Australian dollars) (Editing by Ed Davies)