A Canberra lawyer acting for East Timor in its spying case against Australia has accused ASIO of raiding his office and taking papers relating to the case just days before it’s due to begin.

Bernard Collaery says two agents seized electronic and paper files from his law practice, saying they worked for the Australian Security Intelligence Organisation.

He also fears a key witness may have been arrested.

Mr Collaery told the ABC on Tuesday the agents would not show his employees the search warrant because it related to national security but spent “some hours seizing all manner of documents and other records on the basis that there was a national security issue”.

He is in The Hague working for East Timor which has accused the Australian Secret Intelligence Service (ASIS) of covertly recording East Timorese ministers and officials during delicate oil-and-gas negotiations in Dili in 2004 for the Timor Sea resources treaty.

East Timor is pursuing international arbitration in The Hague to have the 2006 treaty overturned, a process it launched last December after the Australian government failed to respond to its bugging claims.

Mr Collaery told the ABC he also believes a key witness in the Timorese case – a former Australian spy turned whistleblower – has been arrested in a separate raid in Canberra.

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

“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,” he told the ABC.

“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.

“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 said 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.

He also said he has the evidence with him in The Hague so the raid will do little to hinder the case.

Comment was being sought from ASIO.