(Reuters) - Australian Rules club St Kilda Saints are investigating the allegation that one of their players set fire to a dwarf's clothing during their "Mad Monday" celebrations, the Australian Football League (AFL) said on Tuesday.

"Mad Monday" is a traditional day of celebration for Australian athletes in various codes at the end of the season, often involving heavy consumption of alcohol and frequently damaging the image of teams and leagues.

St Kilda, whose season ended on Sunday, included dwarf entertainment in their celebration and one player had set alight the clothing of Blake Johnston, also known as 'Mr Big', his colleague Arthur Serevetas told a local TV station.

"A player went behind my friend with one of those gas lighters that you light up a stove and basically lit him up," Serevetas told Channel Nine.

"Part of his shirt and pants caught on fire. After that someone put it out and he got ticked off and we basically left."

St Kilda said in a statement it had launched a probe into "reports of an incident involving a player at a post-season celebration".

AFL football operations manager Mark Evans said the league was awaiting word from St Kilda once the club had completed its investigations.

"The players have been playing pranks on each other by setting the shoelaces on fire and singing happy birthday and they've singed the clothing of a performer that they had there," Evans said on the AFL's website (www.afl.com.au).

"I think these practices of mad Mondays and bizarre behaviors and all those sorts of things, I think they're well gone.

"It's certainly bizarre, but it's not conceivable that this is the way we should operate at celebrations for (the) end of the season."

AFL chief executive Andrew Demetriou earlier apologized for laughing uncontrollably when confronted with the story on a TV show.

"I thought it was a joke," he told 3AW radio station. "When this was brought up, I just thought it was incredulous to think that this could possibly be. After I found out that it was true, it's just reprehensible. I was flabbergasted."

Demetriou suggested such a prank was the last thing the AFL needed after a season overshadowed by the drug supplements scandal, which resulted in Essendon Bombers being handed the heaviest penalties in the history of the sport.

The Bombers were thrown out of the playoffs, had their coach suspended for 12 months and were fined A$2 million ($1.79 million) after being found to have used their players as guinea pigs in an experimental and possibly illegal supplements regime.

The potent combination of a group of young players coming to the end of a long, hard season and alcohol has caused a string of unsavory "Mad Monday" incidents over the years.

One of the least savory came in 2010 when former Australia rugby league winger Joel Monaghan was sacked by his club Canberra Raiders after a photograph of the player simulating a sex act with a dog was posted on the internet.

(Reporting by Nick Mulvenney; Editing by Ian Ransom)