Attorney-General George Brandis has again defended security service raids on Canberra lawyer Bernard Collaery who was helping East Timor in its case against Australia.

"As the minister who authorised the warrant on the basis of an intelligence case put before me by ASIO that it was a strong case," Senator Brandis told a Senate estimates committee hearing.

Greens Senator Scott Ludlam had asked how raiding the office of a lawyer advising the East Timor government impacted on national security. ASIO director-general David Irvine said he could say nothing about the raids other than that they were approved by the Attorney-General.

Mr Irvine said the object of the warrant was to obtain certain material. "It didn't specify specific documentation relevant to the security matter being investigated," he said.

In December, ASIO executed search warrants on the Canberra homes of an unnamed former Australian Security and Intelligence Service (ASIS) officer and Mr Collaery who is representing East Timor in its spying case against Australia.

East Timor has accused the 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.

The country is pursuing international arbitration in The Hague to have the 2006 treaty overturned.