Police believe the twins, aged 15, were lured to a park off Garretty Road at Caroline Springs, in Melbourne's west, on December 30 after a social media post.
Senior Constable Daniel Rankin said the post suggested a one-on-one fight, but the brothers were instead quickly surrounded by 11 boys, one of them wielding what's known as a "zombie killer" machete.
Police arrived at the park within minutes and the group fled.
Sen Const Rankin said both boys were traumatised by the attack, with one suffering serious injuries after his head was stomped on.
The teenager also received multiple lacerations to his arm and back.
His brother was forced to jump into a nearby lake to get away from the attackers.
"Within a minute they were surrounded. It's a cowardly attack against these two individuals, which has left them with lifelong injuries," he told reporters on Wednesday.
"He is going to have a significant recovery period. (It's a) traumatic event, that's going to leave a lasting impression on his life."
Investigators have released vision of a group of boys aged between 15 and 18 walking through Caroline Springs shortly before the attack.
Police believe seven of the offenders, who were not known to the victims, arrived in the area in a black Ford Ranger ute with false number plates DRY685, which was left parked in nearby Boathouse Drive.
CCTV captured eight alleged offenders leaving in the ute while three others were seen running towards Lorimer Street about 5.45pm.
Victoria remains at the centre of knife crime, with police seizing more than 15,000 knives, machetes and zombie knives in 2025.
Incidents involving machetes tripled between 2021 and 2024, with young males responsible for two in five of the incidents, according to Crime Statistics Agency data.
A machete amnesty, which ended on November 30, resulted in about 25,000 weapons being handed in during the program.
Les Twentyman Foundation chief executive Paul Burke told AAP early prevention was crucial to preventing youth crime.
He hopes Victoria's new Violence Reduction Unit, introduced following a spike in youth crime and taking a preventative, community-focused approach, will help address the ongoing problem.
"Intervening at the earliest possible point for these young people can (break the behaviour)," he says.
Mr Burke said it was becoming increasingly common for young people to start arguments online and take them offline.
"For young people, an online argument isn't virtual. It's personal, it's emotional, and it's very real," he added.
The stabbing occurred several suburbs away from where Dau Akueng, 15, and his 12-year-old friend, Chol Achiek, were stabbed to death in September.
The incident reignited debate over youth gang crime in the city, after the pair were allegedly ambushed by a group while returning home from a local basketball game.
Eight teenagers have since been charged over the crime.
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