The crashes, which happened in Dandenong North about 3.30pm, have caused major traffic delays across Melbourne, with all outbound lanes of the Monash Freeway closed from Eastlink to Stud Road.
One man has been rushed to the Alfred Hospital in a stable condition and two men have been rushed to Dandenong Public Hospital in a stable condition.
The first crash involved one truck and four cars, colliding on Monash Freeway near Stud Road.
Three people were injured in the first crash and were taken to hospital with non-life-threatening conditions.
The second crash, happened very shortly after, just 100 metres up the road, between two tray trucks.
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One driver was injured and taken to hospital in a stable condition.
Police are investigating the cause of both crashes.
The crashes caused major traffic delays, with some cars left stuck in long queues for hours.
Motorists are being urged to get off the freeway at the earliest opportunity and and use the Princes Highway through Dandenong.
The Department of Transport has warned the citybound lanes of the Monash Freeway are also heavily congested past the crash scene.