A kitesurfer has been critically injured after smashing into a car in Porirua, near Wellington.

The man had been testing a new kite when the incident happened at Mana Esplanade this afternoon, a witness told Fairfax.

He was believed to have been knocked unconscious when the ropes of his kite became tangled and the wind picked him up and sent him flying.

The eyewitness said the kitesurfer smashed into the car's bumper, denting it.

He sustained critical injuries and was taken to Wellington Regional Hospital.

The man was being assessed at the emergency department, a hospital spokesman said.