A spectator has been charged after allegedly punching an umpire during the grand final of an amateur football match in Western Australia.

Police in Bunbury, in WA's South-West, charged a 25-year-old man with assault after Sunday's match.

It is alleged the man punched umpire Brenton Bartlett, 24, in the face as he was walking off the ground following a five-point win by the Eaton Boomers over Carey Park in the reserves grand final.

A man at the ground has told the ABC the umpire collected the ball at the end of the game and was running off when he was attacked.

He said the umpire was helped to his feet by trainers, and police moved on to the ground a short time later looking for his attacker.

The South West Football League has also launched an inquiry into the incident.

The league's president Kevin Lynn said there was security at the game at Bunbury's Hands Oval.

"There was a police presence at the ground and this was just something that's beyond anyone's control... when a spectator jumps the fence and assaults an umpire," he said.

"We [the league] will certainly be talking about it, but I just don't know how we stop these sort of people jumping fences and running on to the ground."

Mr Lynn said as far as he was aware, alcohol was not a factor in the incident.

It was the second violent incident involving spectators at football games in Western Australia at the weekend.

A Fremantle fan has also been charged over an incident in which a woman was hit during the Dockers loss to Hawthorn at Subiaco Oval on Friday night.

Cody Yarran, 24, from the Perth suburb of Joondanna, has appeared in court charged with common assault, obstructing a police officer, and disorderly behaviour in a public place.

He was remanded in custody to face court again in October.

The AFL has labelled all violence at its games "completely unacceptable".