'The white van that was involved in the collision is still being sought,' police said in a statement, adding they had set up an emergency call centre for information.
Mobile phones in the area received an emergency alert message warning the public to be on the lookout for a Fiat Doblo with the number 257 as part of its licence plate.
RTL news reported that a trauma helicopter, ambulances and a heavy police presence were at the scene.
Bruno Mars had been the headline act that closed the three-day festival on Sunday night, which also saw Jessie J, Pearl Jam, Foo Fighters, and Noel Gallagher's High Flying Birds perform.
Pinkpop, which has been held annually since 1970, had attracted around 67,000 fans-a-day since opening on Friday, according to director, Jan Smeets.
The organisation said in a statement on social media that it was 'deeply shocked' and added that their 'thoughts are with victims and their families.'