Police had no date or venue for the deadly plot but after an investigation spanning more than 12 months, pounced on the trio during a series of early-morning Tuesday raids across the city's north.

Hanifi Halis, 21, and brothers Samed Eriklioglu, 26, and Ertunc Eriklioglu, 30, later faced court, each charged with one count of preparing or planning for a terrorist act.

Victoria Police Chief Commissioner Graham Ashton said recent monitoring of the men showed they had become "much more energised" in the past week about activating the plot quickly.

"We didn't have a precise time and a precise location," Mr Ashton told reporters.

"They were looking at a place of mass gathering.

"There was a view towards a crowded place, a place where maximum people would be attending, to be able, to kill, we allege, a maximum amount of people as possible.

"I believe, over more recent days, attempts have been made to source a 0.22 semi-automatic rifle to assist with the carrying out of that terrorism event."

Mr Ashton said the accused were using encrypted communications and police acted when they had gathered enough evidence.

All three are Australians of Turkish background and had their passports cancelled by federal agencies in January, March and October.

A man claiming to be the brothers' dad said the pair are "good men".

"They don't even know how to use a weapon," Armagan Eriklioglu told reporters outside his Campbellfield home, earlier raided by officers.

"They are very quiet, they're not wild, they're not aggressive. We haven't got no weapons, they (police) couldn't find anything."

Mr Eriklioglu rejected reports from authorities that the men had been inspired by ISIS to carry out the plot, saying: "No, no, they don't like Islamic State".

AFP Assistant Commissioner Ian McCartney said the arrests were an "excellent result" and the result of a joint counter-terrorism operation involving Victorian and Australian Federal Police and ASIO.

"If we had not acted early in preventing this attack, we will allege the consequences would have been chilling with the potential ... for significant potential loss of human life," he said.

The accused men faced Melbourne Magistrates Court together for a brief appearance during which a prosecutor outlined a mass of evidence including more than 17,000 phone intercepts, 10,500 text messages and 7800 hours of recordings from listening devices.

Premier Daniel Andrews said the plot could have been "potentially deadly" had police and agencies not intervened.

"The seriousness of this potential incident should not be underestimated."

The arrests come as Melbourne hosted a state funeral for terror attack victim Sisto Malaspina. He was killed and two other men were injured on November 9 when 30-year-old Hassan Khalif Shire Ali drove a burning ute into the city and launched a knife attack.

Police shot Shire Ali who later died in hospital.

Australian Associated Press