Police say it is too early to know how a car came to crash into cyclists in Kununurra at the weekend, killing a five-year-old boy.

The white Landcruiser struck the two bicycles as they were being ridden along the Victoria Highway late on Saturday afternoon.

A 37-year-old man and nine-year-old boy were seriously injured and flown to Darwin Hospital for treatment.

The younger boy, who was a passenger on the man's bike, was critically injured and died at the scene.

Detectives worked through the night to examine the scene.

Major Crash investigators will take over the case this week.

Sergeant Adrien Doulis said anyone who saw the vehicle or the bikes should contact them.

"Investigators are calling for witnesses in regards to the crash, or information in regards to the movement of the pushbike or the Toyota Landcruiser prior to the crash," Sergeant Doulis said.

The 56-year-old man driving the Landcruiser was not injured.

"Investigations are continuing and the driver of the Landcruiser has been assisting police with their inquiries," Sergeant Doulils said.