"There have been some raids over the last 24 hours in Sydney and Melbourne. There's been at least one arrest," he said.
"There is evidence of a bomb plot that was in a reasonably advanced state of preparation and the police will have more to say about this."
A smaller raid took place in Sydney, where a 14-year-old boy was arrested, some outlets are reporting.
Victorian acting Deputy Commissioner Robert Hill said this case had been subject to an intense investigation "especially over the last nine days".
"The facts are this: As a male was leaving that address in a motor car with a female he was intercepted by our specialist police," he said.
"The 17-year-old male was taken into custody. The female was secured."
The young man is expected to face a closed court on Monday and his name will not be released.
Police say there is no longer an imminent threat to the community.
Officers found three suspected improvised explosive devices that they "rendered safe" during the course of their investigation of the Greenvale house.
Australian Federal Police Deputy Commissioner Mike Phelan said their number one priority was public safety so their risk tolerance was "extremely low" and they don't always have a lot of information before they decide to act.
He would not confirm speculation the alleged terror plot was aimed at Mother's Day.
"We're sifting through a lot of information ... we can't confirm exactly when any event was going to take place."
But Mr Hill said they believed it was a threat to the Victorian community.
"The level of impact, the level of injury that those suspected devices may have caused is yet to be able to be determined because of the infancy of the investigation and forensic analysis but we believed that that threat was real."
Asked if the boy was working alone, the officers told reporters that they were still sifting through information but there was no longer a threat from the youth, or his group.
There were no links to Operation Rising, in which 200 heavily armed officers stormed properties in the city's southeast on April 18, over an alleged Islamic State-inspired terror plot planned for Anzac Day.
In those raids, five men were arrested and three later charged.
Nor could they confirm at this stage if the raids were linked to any others in any other jurisdiction.
- AAP
