Associate Professor Charles Livingstone, who is a gambling researcher at Monash University's school of public health, said video games used random rewards and "flashing lights and bells and whistles" to keep people playing, just like poker machines.

"All of these types of games and gambling devices use the same principles," Dr Livingstone said. "Gaming is emerging as a new source of grooming people and teaching them how to gamble. This is why the World Health Organisation has a remit from some of its members to look at gaming as a serious addiction."

Digital sales - sales of games and in-game purchases combined - exceeded those of games bought in bricks-and-mortar stores for the first time in 2014 and that trend accelerated last year.

"It's another form of media where people will invest, like Netflix or Spotify," said IGEA chief executive Ron Curry.

Mr Curry said there were tools for parents to restrict in-game spending and the time that children spend playing games. Meanwhile, it was a "perfectly acceptable way" for adults to spend their disposable income.

"It comes back to what you're doing and being mindful that it's not impacting other parts of your life," Mr Curry said. "If you're spending your rent on a game, that's a problem. But if it's part of your considered entertainment spend, it's no different to people buying beer or cigarettes or pokies."

Kyle Duncan, a 30-year-old plant mechanic from Cessnock who plays FIFA soccer games, said he found it easy to rack up a large bill by making regular small purchases, estimating he spent $4000 over a couple of months without realising it.

"It's so easy," Mr Duncan said. "I always had a pretty basic team, but it really took a hold of me this year. I just really didn’t care and it just got a bit out of hand.”

Electronic Arts, the publisher of FIFA, released a report in July that said 28 per cent of total revenue comes from the Ultimate Team game mode, which combines card collecting and playing matches to try to build the best team. The option to spend real money to open packs of cards has turned 'Ultimate Team' into a multi-billion dollar business.

Mr Duncan had played FIFA games for more than 20 years and Ultimate Team for about five years but didn't spend real money on it until this year.

He didn't realise how much he'd spent until his wife added up all the digital receipts from the Xbox store.

"She went through all the emails from Microsoft one day - that was a pretty fun phone call," he said. "But it could have been a lot more if not for her adding it up and bringing it to an end."

Another player, who asked to remain anonymous, said he's spent close to $10,000 over the past few years on Ultimate Team - but defended it as "a bit of fun" and no different to buying a new video game each week.

Fortnite, one of the most popular games among teens, made more than $2 billion worldwide from in-game purchases last year, despite the items not offering any competitive advantage in the game.

Gamers can buy custom dance moves and outfits (or "skins") for their characters. Many see it as a way to support the game, which is free to play.

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Fortnite fan Patrick Daly, 13, from Peakhurst is only allowed to play on weekends but he also watches hours of YouTubers playing the game, and talks about it with his friends each day.

Patrick estimates he’s spent about $200 on the game using birthday gift vouchers and pocket money, with purchases including Raptor and Merry Marauder 'skins'. "They’re no use in the game, but they look cool," he said.

His father Brian, 50, is happy for Patrick to spend his money how he wishes.

"As long as it’s not over the top and he’s not using our cards or that sort of thing, it’s all good," he said.

"I can understand why they do it - all their mates do it and they want to keep up with each other, I guess."

Epic Games (the developer of Fortnite) and EA were contacted for comment.