Mr Shire faced Melbourne Magistrates' Court on Tuesday charged with preparing to commit a terrorist attack and collecting documents to facilitate a terrorist attack.
He had connections with terrorists and he would become "energized" online when learning of overseas terrorist attacks, Patton said.
"The arrest means this threat has been removed, we are quite confident there is no risk or no threat to the Victorian community now we have removed him from society and he is being interviewed".
Tens of thousands of people gather for New Year's Eve celebrations in Federation Square every year.
Patton said the man had connections with extremists but was believed to be acting alone.
Patton described the suspect as "a person who's expressed an intention to try and kill as many people as he could through shooting them".
Love jihad? Hadiya arrives in Delhi for SC hearing amid tight security
The Supreme Court on Monday will hear the Kerala Love Jihad case, with Hadiya alias Akhila present before it. Shafin Jahan had moved the Supreme Court against the Kerala High Court order which annulled their marriage.
The arrest was made after Ali, who police say consulted radical websites and owned Al-Qaida documents for committing terrorist attacks, began face-to-face meetings about getting a weapon.
Patton said the suspect was born in Australia to Somali parents and had been monitored by police as a "person of interest" since January.
Police obtained and executed warrants on Ali's family home in Werribee, a relative's home, and his place of work after he started trying to get a gun. One of the charges he is facing carries a penalty of life in prison.
In a statement, they added that nobody else was expected to be arrested in connection with the alleged plot.
Since September 2014, when the national threat terrorism level was raised to "probable", the AFP has worked with its state and territory partners to thwart a number of plots.
The Melbourne Joint Counter Terrorism Team and the Victoria Police Counter Terrorism Command maintains strong links with the intelligence and other partner agencies which gives us the best possible understanding of emergency issues.