Documents obtained under Freedom of Information laws show the Federal Government knew about the secret US internet spying program PRISM months before a whistleblower made details public.

The Attorney-General's Department prepared a secret briefing about PRISM in March, two months before former National Security Agency (NSA) contractor Edward Snowden leaked information on the global spying program to The Guardian newspaper.

Snowden's revelations showed that the US government had in place a global spying system which taps deep into millions of individuals' lives via social media platforms and email.

It prompted questions about how widely the US government was sharing the information with its allies, including Australia.

Greens Senator Scott Ludlam, who has long been vocal about his concerns concerns over digital privacy rights, says the timeline is interesting.

"The Australian Government has tried to be completely opaque about this," he said.

"Our attorneys-general, either of the Labor or the Liberal variety, will just wave their hands and say 'national security', and that's meant to make you stop asking questions."

The ABC was refused access to the document, titled Protected Brief to the Attorney-General.

Under national security exemptions, .

Interestingly, talking points about what effect PRISM would have on the privacy of Australians was considered too sensitive for public release.

The same goes for an analysis of media reporting on the issue.

The Department's deputy-secretary, Tony Sheehan, says it is because it relates to an intelligence agency document.

Senator Ludlam says that excuse is too common in Australia.

"This is the scandal that's broken open in the United States and the UK and other democracies around the world," he said.

While the FOI documents show a close relationship between Australian and US authorities, what is still not known is how often Australian intelligence and law enforcement agencies make use of data obtained by the NSA.

Sean Rintel, a board member with online liberties and rights organisation Electronic Frontiers Australia, says the Australian Government "knew an awful lot" but and was not doing anything to "at least reassure the Australian public that what it was doing was in its interest".

"There's an awful lot that's not being talked about there," he said.

"And the lack of oversight over these kinds of what we now know to be very-large-scale surveillance operations is the concerning thing about what's going on here.

"The documents were requested from 2007 and yet there's only four and they're very heavily redacted and one was not even allowed."

The Attorney-General's Department was asked to comment on the issue, but has not responded to AM's emails.