A lawyer representing East Timor in its spying case against Australia says his office has been raided by the Australian Security Intelligence Organisation (ASIO).

Bernard Collaery says two agents seized electronic and paper files this afternoon from his law practice in Canberra. He says the agents identified themselves as working for ASIO, and would not show his employees the search warrant because it related to national security.

East Timor has accused the Australia Secret Intelligence Service (ASIS) of covertly recording Timorese ministers and officials during oil-and-gas negotiations in Dili in 2004.

Mr Collaery also believes that a key witness in the Timorese case – a former spy turned whistleblower – has been arrested in a separate raid in Canberra. The ABC is trying to confirm the raid and the arrest.

The negotiations led to the lucrative Certain Maritime Arrangements in the Timor Sea (CMATS) treaty, with both countries agreeing to a 50-50 split of an estimated $40 billion in revenue from the gas development. But the treaty is now under threat, with East Timor launching a case to an arbitration panel at The Hague.

Mr Collaery, who is currently at The Hague as part of his work on the case, told PM that the ASIO agents spent "some hours seizing all manner of documents and other records on the basis that there was a national security issue".

He says the ASIO agents gave his staff no details of their search warrant because it dealt with national security matters.

"I mean how absurd," he said. "I have no way at this moment of knowing the legal basis upon which these unprecedented actions [took place] – raiding my law offices to procure evidence which is about to go on the table in The Hague."

Mr Collaery says the raid is a "blatant, disgraceful attempt" to impede justice being done for East Timor.

"I left Australia just 24 hours ago. There was ample opportunity, I am sure, for the warrant to be executed and for the attempt to be made whilst I was in Australia and could handle the situation," he said.

"This can only relate to the proceedings against Australia over the bugging of the Timor Leste cabinet offices during the negotiations for a petroleum and gas treaty in 2004, so this is a further step in the actions by Australia to shore up its illegally procured treaty."

Mr Collaery says the seized documents include evidence of Australia inserting listening devices into the wall of the East Timor government's cabinet room ahead of the negotiations. However, Mr Collaery says he has the evidence with him in The Hague and the raid will do "very little" to hinder East Timor's case.

"The evidence is here. I can't see what the Government hopes to achieve by this aggressive action," he said. "It can attempt to nullify the whistleblower's evidence, but that evidence has flown – the evidence is here, it's abroad, it's ready."

But he says the raid may act as a measure of intimidation against anyone else who wanted to come forward against the Australian Government.

"What it may of course do is restrict the flow of evidence; it may impede others to come forward who were unfortunately pressed into the service of the minister at the time, Alexander Downer, to effect this bugging operation," he said.

ASIO's official response on the raid is that it has "no comment on the matter".

Last week, the man tipped to be East Timor's next prime minister became the most prominent leader to go public with the accusation against Australia. Agio Pereira says the bugging took place and it gave Australia a massive advantage.

"Insider trading in Australia is a crime. And when you bug the negotiating team's evaluation of the impact of their negotiations, you do have an advantage," he said last week.

"It's more than unfair, it actually creates incredible disadvantage to the other side and according to international law, the Vienna Convention and the law of treaties, you're supposed to negotiate in good faith."

"It's not about money; it's about sovereignty. It's about certainty, and it's about the future of our future generations. It's very important for Timor."