The Government has agreed to an Opposition demand to modify its data retention legislation, to better protect journalists' sources.

Prime Minister Tony Abbott has offered to amend the bill, so that agencies that want to access journalists' metadata for the purpose of identifying their sources will now require a warrant to do so.

In a letter to Opposition Leader Bill Shorten, Mr Abbott stated that while the Government "does not believe that this is necessary" he would propose "to expedite the bill".

"I have decided that a further amendment be moved that will require agencies to require a warrant," Mr Abbott wrote.

He said that access to metadata remained "absolutely crucial" to the capacity of security agencies to fight terrorism and serious crime.

In exchange, the Government wants Labor's help to pass the amended legislation by the end of next week, including extended sitting hours and agreeing to scrap an additional inquiry into journalists' source protection.

Mr Shorten said he wanted to see the amendment before deciding whether or not to support it.

The Government's legislation would see telecommunications companies required to retain some data from their customers for two years.