The Federal Government's proposed data retention regime would serve to combat authorities' "eroding" ability to access communications data in investigations, Australia's key spy organisation has argued.

In a surprising submission to the joint parliamentary committee investigating the proposed national security reforms, ASIO said it sought to clarify the type of data telcos and Internet Service Providers would be required to hold for up to two years under such a scheme.

The metadata, which it termed "communication assisted data", would include the user's identification; origin and destination of the communication; time, date and duration; type of communication and device used; as well as the location of the user during that communication.

"In this context, agencies are not seeking access to the content of communications," ASIO argued (pdf), though it noted it could separately access such content using certain warrants.

The submission provides the first concrete evidence of the data authorities hope to gain under a data retention regime.

The Government has previously asked ISPs to hold onto metadata as part of closed-door meetings held two years ago but carriers have voiced confusion over whether the current proposals align with those requests.

ASIO argued the proposal would follow in line with other countries, particularly those in Europe, who have implemented data retention regimes obliging telcos to create and retain the same types of data ASIO historically did (but no longer does for commercial purposes).

"This data is vital to law enforcement and security intelligence agencies for pre-warrant checks, investigative leads, intelligence and evidentiary corroboration, etc. It may also be used in evidence," the organisation argued.

The organisation's submission, which became available late Thursday afternoon, coincided with a "clarifying letter" (pdf) from Attorney-General Nicola Roxon, who has come under fire in recent weeks over inconsistent comments on her view of the proposal and what it may include.

She argued that the Australian Government's proposals would cause the country to fall in line with the European Union's existing data retention regime, which similarly calls on telcos to hold specific datasets for up to two years.

Despite this, she said she did "not have a specific data retention model in mind".

"For Australia, the principal argument in favour of a data retention scheme is to maintain our agencies' access to a critically important source of intelligence and evidence," she said.

"Agencies have indicated that the need to access this information is immediate and that the [erosion] of such access is already seriously affecting agency investigations."

In separate submissions to the inquiry, Western Australia Police supported the regime, while Victoria Police called for more consistency in the length of time such data is held by telcos.

"Investigations should not be hampered by the unavailability of data from one provider that would have been available had another provider been used," Victoria Police argued (pdf).

"Often, the incoming data from calls/SMS are crucial to the investigation of crimes such as stalking and breaching family violence intervention orders.

"These investigations are being hampered, or in many cases are unable to progress at all, due to the purging of such information from carrier's systems at the earliest opportunity."

It noted the importance of the security of that information being held but said "law enforcement should have no role to play in this due to a clear conflict of interest".