Christchurch police are seeking witnesses to an assault on Riccarton Road where a man was threatened with a metal bar then hit by a van.

Police said at 8:40 this morning the driver of a white van was reported to have approached the driver of a transport truck and threatened him with a metal bar.

Acting senior sergeant Melanie Noonan said the van driver returned to his vehicle and while leaving the scene hit the other driver who had gotten out of his truck.

"Police understand that photographs of the incident have been posted on Facebook and urge the public to think before commenting as this could affect our ongoing investigation into this incident."

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The truck driver was treated at the scene.

Police say the van driver did not remain at the scene, but would be interviewed tomorrow.

Anyone who witnessed the incident or has any information or images is asked to contact Christchurch Central Police Station by calling 03 363 7400.

Information can also be provided anonymously by calling Crimestoppers on 0800 555 111 or online at or online at www.crimestoppers-nz.org