THE world's most popular free online game has been accused of luring children into gambling after a Sydney boy bet more than $2000.

The 15-year-old from Quakers Hill, in northwest Sydney, was playing RuneScape, a game with more than 200 million users, allegedly infiltrated by crime "clans".

His mother, who asked not to be named, was shocked to see a $1200 bill on her credit card. Her son also spent $700 gambling on RuneScape on his mobile phone.

Video Game Addiction Treatment Clinic psychologist Emil Hodzic has had four Sydney teenage patients who gambled real money in games.

"I am seeing a number of teenagers presenting with video game addiction with a co-morbidity of problem gambling behaviour," he said.

"The parents aren't aware their children are also gambling and some owe 'clan leaders' real money, which is part of the reason they're stealing from their parents."

RuneScape players complete medieval quests for points, to buy weapons and food in-game.

Gambling clans also invite players to use real money, Mr Hodzic said.

"These clans are equivalent to a casino. When game developers find out about it they shut them down, but another springs up," he said.

"Parents need to be aware that RuneScape is not as safe and soft as people imagine it to be."

The teenager, who is now undergoing treatment for his online addiction, was also lured into recruiting for the clan, which offered him a 30 per cent commission.

"My son was encouraging others to do it and the clan leader was getting 70 per cent commission," his mother said.

"I know he spends a lot of time on the computer, he was getting up in the middle of the night to play, but I didn't realise there was a dark side."

RuneScape's developer Jagex Games lifted its restrictions on real-money trading last year.

"Before free trade, I played RuneScape the way it was supposed to be played," one player confessed on the online forum sythe.org.

"But ever since the free-trade update I can't stop gambling," the gamer said.

The company warns of gambling scams on its RuneScape HQ forum, saying advertising RuneScape gambling games will not be allowed.

"Some players will attempt to build your trust in order to scam a larger portion of your wealth off you," it warns players. "Be careful." allowed an 'explosion' of gambling the site since the Jagex removed the trade limits last year"Kids are definitely being exposed to gambling without their parents' knowledge.

"Criminal gangs have been involved in other online gambling games so it should surprise no one that they may be targeting kids on RuneScape."

Earlier this year, the Australian Law Reform Commission handed down its review into the classification system.

"All online games remain exempt from classification for two years while the whilst the recommendations from the ALRC review are considered," a spokesperson for Justice Minister Jason Clare said.