Western Australia senator Scott Ludlam has labelled Attorney-General George Brandis "weak" and told Australians not to trust the government and law enforcement agencies when it comes to trawling through their metadata.

"We should not trust authorities, we should not automatically trust the police," the Greens senator said at a privacy forum in Melbourne on Friday.

Greens senator Scott Ludlam said the Attorney-General George Brandis was "weak". Credit:Hannah Francis

"We should not trust government – God knows, having spent a little bit of time on [Canberra's] capital hill, don't trust these people."

The final of three tranches of amendments to national security legislation, expected to be introduced at the end of October, would force telecommunications companies to retain customer metadata for up to two years.