A cyclist who hitched a free ride up Ngauranga Gorge on the back of a truck could face a careless driving charge if police catch up with them.

On Monday afternoon, a motorist snapped a photograph of what appeared to be a person on a bicycle latched on to a truck heading up the steep gorge during rush hour.

The stunt has been labelled a "very dangerous activity" by police, who are asking for help to identify the rider.

Senior Sergeant James McKay said the cyclist was putting himself at risk of death or serious injury from following motorists if he fell off.

He was also putting the following motorists at risk of crashing and serious injury if they had to attempt to avoid him in the event of a fall, McKay said.

"The cyclist could face a charge of careless driving, as they were in charge of a vehicle, being a bicycle, so if anyone has information on who the cyclist may be, we would like to hear from them."

The cyclist's towing tactics - one hand holding onto the tailgate of the small truck and one leg resting on the lower bumper - were snapped by a passing motorist who labelled the action as "incredible".

"We were driving up the gorge and noticed the truck with a cyclist hanging off," the motorist said.

"I'm not sure if the truck driver noticed he had a cyclist hanging on or not."

The speed limit along the busy main route north out of Wellington is 80kmh.

The motorist said the truck would have been doing between 60 and 70kmh and the rider clung on the whole way up the gorge.

"At that speed during peak-hour traffic, I wouldn't say it was a wise move," she said.