WA police have released pictures of a man they believe jumped from a moving train into the Swan River last weekend, in what has been labelled a "gobsmacking" and "idiotic" stunt.

The images of the shirtless man were taken at the North Fremantle train station just before he jumped from the roof of a train as it crossed the Fremantle rail bridge about 10.30am on Saturday, narrowly missing live overhead wires carrying 25,000 volts of electricity.

The man climbed on to the train at North Fremantle station. ( Supplied: WA Police )

He plunged several metres into the water, narrowly missing one of the bridge's concrete pylons.

The stunt was recorded by someone on the riverbank and came to light after the footage was posted on social media.

Police say CCTV cameras at the North Fremantle train station recorded the man walking north on Stirling Highway before the incident.

He then loitered in the car park for several minutes before jumping over a fence, crossing the railway tracks and climbing up the outside of a Fremantle-bound train.

The man is described as being aged between 25 and 35, with ginger hair, light skin and of medium build, and at the time was wearing only dark-coloured shorts and sneakers.

Police say the man could be charged with trespass, and face up to 12 month's jail. ( Supplied: WA police )

Police say he could be charged with trespass, which carries a maximum penalty of up to 12 months' jail.

The stunt was condemned by the Public Transport Authority who labelled it "gobsmacking" and "jaw dropping," saying the man was lucky not to have been killed or injured.

WA Premer Mark McGowan described the incident as "idiotic".

Anyone with information about the matter is asked to contact Crime stoppers.