Wellington - Two men died while apparently joy-riding on the roof of a freight train in the early hours of Thursday, according to police in New Zealand.

The bodies of the pair, believed to be in their 20s and with severe head injuries, were found after the train arrived in Christchurch from the South Island port of Picton.

"What these two young men appear to have been doing was a very dangerous thing," Detective John Rae told The Press newspaper's website. "It was fatal for them."

He said it was believed they had hit a bridge as the train passed under. (dpa)