Crusaders and All Black lock Ali Williams has turned from rugby hero to sheriff by wading in and breaking up a fight in Christchurch.

Williams and a couple of Crusaders teammates were out celebrating their team's Super 14 semifinal win over the Hurricanes on Saturday night, when they came upon a fracas in the inner city.

Williams' actions were captured on film and shown on One News on Tuesday night.

The big lock, no stranger to off-field misbehaviour himself, is shown getting in amongst a group of five or six brawling young men, and stopping the fight.

Max Rooney, the man who filmed Williams on his mobile phone, said there were a group of people aged about 20 to 25 arguing and then a brawl broke out.

"Then Ali Williams rushed up and saved the day," he said.

Afterward he posed for a picture with Rooney's girlfriend.

"Ali Williams is, like, renowned ... but this time he just, like, handled it real well," Rooney said.

Williams played down his role, saying anyone else would have done the same.

The big lock ended six seasons with the Auckland Blues in controversial fashion last year, walking away from the franchise following a rift with coach David Nucifora.

Williams was sent home from South Africa ahead of the Blues' semifinal as punishment for off-field misdemeanours.

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