IT’S the great 7-Eleven coffee cup mystery of 2018.

The nation’s second-largest coffee chain has been accused of overcharging customers by 50 to 100 per cent with its $2 and $3 “large” and “super” cups.

Melbourne filmmaker Tommy Jackett blew the lid off the issue in an Instagram video, showing that the contents of a $3 cup easily fit into a $2, while a $2 cup “almost” fits into a $1 cup.

“I love @7elevenaus coffee, so you can imagine my upset, confusion and OUTRAGE when I discovered their $3 coffee SCAM,” Jackett wrote. “Shout out to @ryan.jon for helping me blow this wide open. #CoffeeSizeMatters.”

The former breakfast radio announcer, who now runs a media company and hosts The Daily Talk Show podcast, said he went down the 7-Eleven coffee “rabbit hole” when he noticed a line out the door on Bourke Street of people waiting to use the machine.

“It blew me away how big it was getting,” he said. Jackett said he and his co-host “talk a lot about 7-Eleven coffee” on their podcast. “I drink long blacks from there all the time.”

He loves the coffee so much he even grabbed a bag out of a 7-Eleven bin to find out where it comes from.

“They’ve got their own blend of coffee, I saw the address of where they roast it in Preston,” he said.

“I went to the coffee factory and asked them if I could buy a bag. They said they don’t sell it but I got to talking with a lady who works there. It turns out the same place that does the 7-Eleven coffee also roast the McDonald’s beans.”

A 7-Eleven spokesman said the convenience chain’s coffee was “outstanding value for customers and the simple $1, $2, $3 price points are based around industry standard cup sizes”.

“Depending on the product the customer selects, there are different ratios of coffee, hot milk, and milk foam — proportionately more for each cup size — and of course the foam does settle over time, or when being repoured as in this example,” he said.

“One of the reasons we introduced other sizes is because our customers told us they wanted the convenience of a range of sizes to choose from. But customers are welcome to super-size the value they get from our coffee by purchasing three $1 coffees instead of the $3 size.”

He added that 7-Eleven would “also encourage the filmmakers, and all our customers, to use our in-store cup recycling units and rescue their cups from ending up in landfill”.

According to 2017 Square Australian Coffee Report, the national average price for a regular-size cappuccino — typically eight-ounces, or 240mL — is $4.10.

McDonald’s is the country’s largest coffee chain, selling roughly 84 million cups a coffee a year, or somewhere between 144,000 and 250,000 per day.

Cheap coffee has emerged as a major convenience battleground in recent years, with Coles Express recently launching 80-cent coffee to try to undercut 7-Eleven.

frank.chung@news.com.au