Detectives from the Heavy Vehicle Crime Investigation Unit allege a 16-year-old Brighton teenager fitted the truck out with red and blue flashing lights, a spotlight on the rear of the vehicle, and even installed an emergency siren.

Police became aware of the bogus emergency truck after seeing online footage of the truck weaving through traffic with emergency lights flashing.

Detectives allege the truck was modified so convincingly, frightened motorists were pulling over to the side of the road as it sped past with its siren blaring.

Police said the teenager - who is a learner driver - even responded to car crashes, and in some instances got out of the truck to help the people involved in the accidents.

"He doesn't have any qualifications that would allow him to breach the road rules so it's a bit of a recipe for disaster," Senior Constable Fritz Ebert said on Tuesday.

Police have estimated the vehicle, and all the modifications made to it, would have cost more than $100,000.

The truck was mostly driven around the Melbourne suburbs of St Kilda and Elwood.

Victoria Police said the teenager was arrested on Tuesday afternoon and is assisting them with their inquiries.

The teenager is expected to be charged with reckless conduct endangering people.

Police are also expected to speak with his parents.

The truck has been seized by police.