Senior federal police say their ability to investigate major crime will be ''devastated'' if a controversial proposal to track the internet use of Australians does not go ahead.
And the Australian Federal Police have asked for the public ''to trust us'' to use the online data responsibly, saying police surveillance would be subject to considerable internal and external oversight.
The contentious proposal to force telecommunications companies and internet service providers to retain user data for two years is the subject of a parliamentary inquiry.
It has raised fears over the privacy and individual liberties of internet users, with critics arguing the regime would allow mass surveillance of the population.
But the police say their ability to investigate and track criminals will be hampered without some form of online data retention.
The AFP are among a host of law enforcement agencies, including ASIO and the Australian Securities and Investments Commission, pushing for internet data retention.
The head of the AFP's high tech crime operations, Neil Gaughan, said the force had a good record with not abusing its powers, and was subject to ''significant'' internal and external oversight.
''I know it's a big ask, but we're asking people to trust us,'' Assistant Commissioner Gaughan said.
''We won't hold the information, it will be held by the ISPs or the telcos, and we require an authorisation by a superintendent of police or above to obtain that information,'' he said.
''To be blunt, we are too busy policing the community and policing national and international law to go on fishing expeditions. We just don't have the resources.''
Assistant Commissioner Gaughan said police were losing the capability to track and investigate crime as communications moved online and away from fixed lines.
The AFP are expected to appear before the Senate estimates committee today, and may be quizzed on the mandatory data retention scheme.
Greens Senator Scott Ludlam, a fierce critic of the plan, said the AFP's argument that it would lose investigative capability did not stack up.
''I don't buy it at all,'' Mr Ludlam said. ''We're discussing categories of data that didn't even exist five years ago, they're not losing anything.
''They were never able to access details on latitude and longitude of every Australian citizen at every time of day.''
The mandatory data retention plan is being considered with a package of more than 40 changes to national security legislation.
The changes are the subject of an inquiry by the joint parliamentary committee on intelligence and security.
Hundreds of submissions were received on the proposed changes before August 20.
The inquiry is expected to report back to the federal government by the end of this year.