A first-hand witness to alleged Australian spying against Timor-Leste in 2004 has been detained and searched at his Canberra home, according to a prominent human rights lawyer and academic, Frank Brennan.

Brennan told Guardian Australia the senior retired Australian Secret Intelligence Service (ASIS) agent, who is a prime witness in the Timorese espionage case against Australia in the international courts, was detained and searched, as was his wife.

He told the ABC the whistleblower intended to provide “credible direct evidence” of the bugging of the Timorese cabinet rooms in 2004. Brennan described the Australian government’s actions as “cowboy antics”.

A lawyer at the heart of those proceedings, which centre on the negotiations about a billion-dollar oil and gas treaty in 2004, also had his Canberra law office raided by two agents from the Australian Security Intelligence Organisation.

Bernard Collaery told the ABC on Tuesday evening his senior law clerk had informed him two agents identifying themselves as ASIO had raided his office while he is in the Hague. Collaery is seeking witness protection in the Hague for the former ASIS whistleblower.

“As I understand it, agents of ASIO executed a search warrant on my law practice and spent some hours there seizing all manner of documents and other records on the basis there was a national security issue,” Collaery told the ABC.

He suggested the raid “may be an intimidatory gesture towards others” inclined to assist in the Timor case, which involves allegations Australia listened in during high-level negotiations over the lucrative oil and gas treaty.

Collaery told the ABC he believed the key witness had been arrested in Canberra – the former intelligence official who had come forward as a whistleblower in the Timor case.

“The agents who effected the warrant refused to give to my senior law clerk of my practice a copy of the warrant, saying it contained national security secrets,” Collaery told the ABC of the afternoon’s events.

“I mean how absurd. I have no way of knowing at this moment the legal basis for this unprecedented action of raiding my law offices to procure evidence which is about to go on the table in the Hague.”

Brennan said young women in Collaery’s office felt intimidated during the search.

Timor-Leste was preparing to present an international court with new evidence of spying during the critical 2004 negotiations over a gas treaty.

Alexander Downer, who was foreign minister at the time, accused Timor-Leste of an opportunistic publicity stunt by renewing its claims, but Agio Pereira, the president of the Timor Leste council of ministers, said the evidence was compelling.

International arbitrators have been appointed to review the 2006 treaty amid concerns of commercial advantage gained by Australia's alleged eavesdropping.

Timor-Leste says Australia has failed to provide an explanation for the allegations.

The attorney-general, George Brandis, issued a statement late on Tuesday night: "I confirm that today, ASIO executed search warrants at addresses in Canberra, and documents and electronic media were taken into possession."

"The warrants were issued by me on the grounds that the documents contained intelligence related to security matters," Brandis said.

"I have seen reports this evening containing allegations that the warrants were issued in order to affect or impede the current arbitration between Australia and Timor-Leste at the Hague. Those allegations are wrong."

"I have instructed ASIO that the material taken into possession is not under any circumstances to be communicated to those conducting those proceedings on behalf of Australia."