The Australian Security and Intelligence Organisation joined police in carrying out search warrants on three addresses linked to Monday's attack in Australia's second-largest city, Victoria state Deputy Police Commissioner Shane Patton said.
Police shot dead gunman Yacqub Khayre, who they said had a long criminal history, on Monday night after he killed a man in an apartment block in a beachside Melbourne suburb and held a woman hostage for several hours.
Three police officers were wounded in a shootout at the end of the siege but the woman was unhurt.
Patton told reporters a 32-year-old man was arrested after Friday's raids. A second man, 31, and his father were being questioned by counter-terrorism police, he said.
The men were not suspected of terrorist activity but "they may be involved in the provision of firearms in this matter", Patton said.
Police seized an imitation shotgun but no firearms or live ammunition were found, he said.
A Reuters witness saw five uniformed and plain-clothed police wearing gloves and breathing masks entering an apartment in a three-storey block and putting household items into plastic evidence bags.
Monday's siege sparked debates about immigration and parole laws. Pauline Hanson, leader of the far-right One Nation party, has said she would support a travel ban similar to those US President Donald Trump has tried to introduce, as well as the internment of people on security watch lists.
Khayre was granted parole in November after being convicted of a violent home invasion. Prime Minister Malcolm Turnbull said parole laws would be discussed at an annual meeting with state leaders in southern Tasmania on Friday.
"If you have someone who has terrorist sympathies and who has a propensity to violence, every day they are not on the street is a good day," Turnbull told reporters.
"I look forward to the first ministers today agreeing that there will be a strong presumption against bail or parole for persons who have shown support for, or have had links to, terrorism or violent extremism," he said.
Khayre was also acquitted of a plot to attack a Sydney army base in 2009.
(Additional reporting by Jane Wardell in SYDNEY and Charlotte Greenfield in WELLINGTON; Writing by Byron Kaye in SYDNEY; Editing by Peter Cooney, Chris Reese and Paul Tait)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.