Uncle Robbie Thorpe has been pursuing legal action against the king since 2023 over the alleged genocide committed against Aboriginal people.
On Monday, an application to appeal a decision preventing the case from proceeding was denied.
Despite the loss, the Krauatungalung Elder and uncle of Senator Lidia Thorpe said the effort has been worth it.
He vowed to take the fight to an international jurisdiction
"Australia is just totally in denial," he told AAP.
"If it wasn't for the acts of terror and the policies of genocide, Australia wouldn't exist.
"That's my reality, and a lot of other people's."
Mr Thorpe filed a charge sheet with the Magistrates' Court of Victoria in 2023, naming himself as the informant and the King as the accused.
The offences alleged were continuing acts of genocide committed since September 28, 2022, against First Peoples, contrary to common law, federal law concerning genocide and First Peoples' sovereign laws.
But the filing was knocked back by a senior registrar of the Magistrates' Court on the grounds that it did not reference any "proper source of law".
A judicial review by the Supreme Court upheld the registrar's decision.
The Court of Appeal dismissed an application by Mr Thorpe to appeal the decision on the basis it had no real prospect of success.
"While acknowledging the significance of the questions raised by Uncle Robbie, the primary judge stated (correctly and self-evidently) that the only jurisdiction she could exercise was the jurisdiction of the Supreme Court of Victoria," the appeal court said.
"The primary judge concluded that the senior registrar of the Magistrates' Court acted lawfully ... as the charges alleged in the charge-sheet did not disclose an offence known to the law of Victoria."
Mr Thorpe said his efforts in domestic courts would play a role if the case ever reaches an international jurisdiction.
"We're exhausting the domestic remedies," he said.
"It's accruing all this unwillingness, inability, reluctance to deal with these issues.
"Sooner or later, we'll get a chance in the international court, and this will all be relevant."
Mr Thorpe suggested Turkey might be the country to take the charges to an international court but said there were a lot of people that "don't like" Australia.
The Indigenous activist has a long history of using the courts in a bid to have authorities admit to genocide.
He tried to charge the Commonwealth with attempted genocide and war crimes against indigenous Australians in the 1990s.
In 2006, he hand-delivered a letter to a residence hosting Queen Elizabeth II, calling on the monarch to implement a treaty and address issues of genocide and sovereignty.
He also co-founded Camp Sovereignty, a long-running Aboriginal protest site in King's Domain, Melbourne.
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