He said authorities have eliminated the threat of the attack, saying "the risk that was posed by this person has been removed".
Justice Minister Michael Keenan said police arrested four extremists last December over a similar plot to attack Federal Square and other public places in Melbourne over the Christmas-New Year period. The four will stand trial next year on terrorism charges.
Keenan urged the public to continue with their usual routines over the period and expect to see a heavy police presence.
"The aim of these terrorists is to foster fear and intimidation, but Australians should be able to go about their business secure in the understanding that the Government has worked very closely with law enforcement and other security agencies to keep them safe," Keenan told reporters.
Counterterrorism police had been monitoring the latest suspect, a known extremist and a sympathiser of Isis (Islamic State), this year and detained him as he met with people in an attempt to acquire a gun, Patton said.
Australia has strict gun laws and automatic weapons are banned from private ownership.
The man is the 74th suspect arrested in Australia in 31 counterterrorism investigations since Australia's terrorism threat level was elevated to "probable" in September 2014.
The plot was the 14th that police say they had disrupted since the terrorist threat level was raised. Five plots have been executed.
Ali appeared in a Melbourne court yesterday on charges of preparing to commit a terrorist attack and collecting documents to facilitate a terrorist act. People convicted of those crimes in Australia face a maximum penalty of life in prison. Ali was not required to plea to the charges and did not apply to be released on bail.
Search warrants were issued on Monday at a home in a suburb where the suspect lived with his parents and siblings, at a relative's address in another suburb, and at a computer business where he once had a part-time job.
Recent foiled terror plots in Australia
New Year's Eve, 2017: Ali Ali charged with planning to shoot revellers at Melbourne's Federation Square.
Sydney plane plot, July 2017: Police foil a plot to bring down an aircraft using an improvised device in Sydney. Three men charged.
Christmas 2016: Two men allegedly days away from carrying out a Christmas Day attack targeting Melbourne's Flinders Street Station, Federation Square and St Paul's Cathedral.
Terror teens, Oct 2016: Two 16-year-old boys charged with allegedly preparing a terrorist act and being members of a terrorist organisation after they were arrested carrying bayonet-type knives near a Western Sydney prayer hall.
IEDs in Melbourne, Aug 2016: Counterterrorism police disrupt an alleged domestic terrorist attack in Melbourne, potentially involving improvised explosive devices.
Attack map, June 2016: A 17-year-old Sydney teenager with special needs was charged after allegedly posting threats online and creating an "attack map" in preparation for a terrorist attack.
Government building plot, May 2016: An 18-year-old man arrested for allegedly planning a terrorist attack on a government or police building in Sydney.
Naval base target, Dec 2015: Two men charged for preparing a terrorist attack, Sydney's Woolloomooloo naval base allegedly a potential target.
Five charged, Dec 2015: Five people charged with conspiracy to conduct an act in preparation for a terrorist act, with three already in custody and a 15-year-old and 20-year-old arrested at their NSW homes.
Homemade bombs, May 2015: A 17-year-old boy charged with planning a terrorist attack in Victoria using homemade bombs following a tip-off.
Anzac Day 2015: Three Melbourne men charged over a plan to behead a police officer on Anzac Day.
Temple firebomb plot, Feb 2015: Counterterrorism police raided the home of Isis supporters Omar Al- Kutobi, 24, and Mohammad Kiad, 25 in Western Sydney. Both men were jailed for at least 15 years over the planned firebombing of a Shia temple in Western Sydney.
- AP, AAP
