Mariah Carey plays supportive girlfriend as she accompanied boyfriend, billionaire James Packer, in launching the Studio City casino resort in Macau on Tuesday.

The casino project, with the Australian owning majority of the $4.4 billion investment, is themed after Hollywood which is why A-listers Leonardo DiCaprio, Robert de Niro and Martin Scorsese where there in attendance.

Reports claimed that the three were paid a total of $17 million to endorse the casino.

Mariah Carey, who divorced with Nick Cannon last year, posed for photographs at the red carpet event as she locked hands with her boyfriend.

"Mariah, who is scheduled to perform on the night, ensured she caught the eye by fully displaying her cleavage in a plunging black evening gown while standing alongside the mogul," Daily Mail reported.

"With lattice lacing and a fitted upper half that clung to the singer's voluptuous frame, the busty gown ensured she was centre of attention on a night primarily dedicated to Macau's latest gambling hub," it added.

James Packer, meanwhile, kept it simple with a black tuxedo as his 6'6" frame towered over everybody.

The Australian is reportedly worth about $4 billion, which he can trace to his casino ventures after severing his ties to Consolidated Media Holdings, which was a family business, and he set up Crown Ltd.

Back in June, TMZ reported that Mariah Carey and boyfriend James Packer were already talking marriage.

"Our Mariah sources say it's insane ... they've been 'dating' for just a matter of days, but he's already told her he wants to marry her, sooner rather than later," it said.

"The soonest can be when she's officially divorced from Nick Cannon. But we're told Mariah is open to tying the knot with James."

Mariah Carey has been gushing about her boyfriend for a while now, even dedicating "Hero" to James Packer during a performance in the Caesars Palace Colosseum in Las Vegas in July.