The ABC is challenging the warrant on four bases,including that the decision to grant the search warrant,made by a Local Court registrar,fell foul of the implied freedom of political communication in the Commonwealth Constitution.

Michael Rippon,a solicitor in the ABC's legal department,gave evidence on Monday that federal police agent Ian Brumby telephoned him in January and indicated police believed"they had enough evidence"to obtain a search warrant and"they were looking for us to assist them with that".

He said Mr Brumby wanted the ABC to agree to a regime under which the broadcaster would be given"a number of weeks to fulfil the search"for documents within the scope of the warrant and"get back to them".

Mr Rippon said he had a further conversation with Mr Brumby in February in which the federal police agent said"words to the effect of'it's just our job to investigate'"and"we don't want any sensationalist headlines like AFP raids ABC".

The ABC declined to assist in those terms and federal police raided the broadcaster's Sydney headquarters on June 5,a day after they executed a separate search warrant on News Corp journalist Annika Smethurst's Canberra home over an unrelated story.

Mr Rippon said he had concerns during the course of the raid that some of the searches carried out by the federal police for ABC files were"way too wide",including references to Mr Oakes and Mr Clark with"no time period"specified.

The search warrant also referred in general terms to"documents classified as'secret'".

He said he also had a"very uncomfortable conversation"with a federal police agent who said"they were aware that there was ... material on the ABC's website other thanThe Afghan Files which they were concerned about".

"In essence,he was conveying to me that I might want to think about those other stories,"he said.

Mr Rippon said he replied that the ABC published a lot of public interest stories and"I can't see them taking them down".

Matthew Ricketson,a professor of journalism at Deakin University in Melbourne,prepared an expert report for the ABC and also gave evidence on Monday.

Neil Williams,SC,for the federal police,put it to Professor Ricketson that"no journalist can ever promise secrecy to a source".

"How so?"Professor Ricketson replied. He agreed journalists were compelled to reveal their sources in some cases under threat of a prison sentence but said he could only think of one case"in the last decade or so"where a journalist had done so in these circumstances.

He agreed with Mr Williams that journalists had to"think carefully before granting anonymity to any old source".

David William McBride,a former military lawyer,haspreviously admitted leaking material to the ABC that formed the basis of its reports and has been charged with a range of criminal offences.

The hearing continues.