Police believe a group of teenagers with kitchen knives is responsible for armed robberies on four supermarkets in Melbourne's south-east on Saturday afternoon.

Supermarkets in Cheltenham, Brighton, Mount Waverley and Camberwell were targeted in the robberies, with the offenders punching three employees and stealing cash and cigarettes.

Inspector Scott Dwyer, speaking outside Moorabbin police station, said it was believed the offenders were aged between 14 and 17, and were known to police.

"What we can be confident in is that multiple arrests will be made quickly," Inspector Dwyer said.

Police said at least two of the teenagers were carrying knives during the incidents.

The first robbery took place about 3:50pm at a supermarket in Bernard Street, Cheltenham. The offenders then made their way to another supermarket in Cochrane Street, Brighton, where one person received a minor cut to the hand.

The  then Andrew Street, Mount Waverley, before heading to a fourth supermarket in Through Road, Camberwell.

Offenders were driving stolen car, police say

Inspector Dwyer said the youths were driving a white Mercedes, which was stolen in a non-confrontational aggravated burglary in Doncaster East on June 7.

"In that instance the offenders have attended, entered via an unlocked door and then taken the keys before stealing the vehicle," he said. The Mercedes's number plates were covered by a set of plates from another vehicle.

The incidents on Saturday, particularly the assault on three staff members, would have been frightening for those present, Inspector Dwyer said.

"I would like the business owners, and the staff members in particular, and the families of those staff members, to know that we will arrest the people responsible for this. We take this crime really seriously."

He assured the community that these types of incidents were few and far between, and that police had made significant progress with the investigation.

Inspector Dwyer said two Victoria Police operations — Operation Trinity and Operation Alliance, which both target youth crime — had resulted in almost 3,000 arrests across the state in the past 12 months.

"To put that in perspective, that is eight youth gang members or child offenders [a day], for breaking into houses, stealing cars or involved in gang activity."

He said the methodology in targeting youth crime would continue to allow police to make speedy arrests. 

"We know them, we know their methodology – we know exactly who they hang out with."

'Sending a very strong deterrence message'

Victorian Premier Jacinta Allan on Sunday acknowledged the work police were doing in combating youth crime.

"It's about capturing this behaviour but it is too, about sending a very strong deterrence message as well."

She would not directly comment on a Youth Justice Bill that was due to come before parliament this week, but said the reforms would look at the causes of youth crime as well as ways to support people away from criminal activity.

"[It is] reform that is needed to both prevent and deter young people from engaging in criminal behaviour, focusing too on community safety and taking a strong approach where there is criminal behaviour."

Ms Allan said the government would take a "tough approach" when it was needed and that a trial of electronic monitoring had been flagged.