The incident took place on Main Road at Riddells Creek, 55km north of Melbourne about 2.20pm on Monday.
A Victoria Police spokeswoman confirmed an adult, who is yet to be formally identified, died at the scene.
"A child was injured and has been taken to hospital with serious but non-life threatening injuries," she said.
"The exact circumstances are being investigated."
Footage from the scene shows emergency vehicles including fire trucks and ambulances strewn across the thoroughfare.
The truck, also seen in the images, is believed to be a water carrier.
Ambulance Victoria confirmed paramedics had taken the child to Royal Children's Hospital with an upper body injury but in a stable condition.
The truck driver has been taken by ambulance to Royal Melbourne Hospital in a stable condition.
The injured driver of the tanker was taken to hospital where they are in a stable condition. (HANDOUT/ABC NEWS)
Victorian health minister and member for Macedon Mary-Anne Thomas thanked first responders who helped the injured.
"This is a terrible incident and our thoughts are with the family of the victims and the broader Riddells Creek community," she said.
The accident is less than two weeks after 11-year-old Jack Davey was killed and four other students injured after an SUV crashed through a fence at Auburn South Primary School in Melbourne's east.
A private funeral was held for Jack on Sunday.
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