Organisers had been expecting about 2,500 people to turn out for the mass nude photo on the steps of the iconic Australian landmark.
The number of people who turned out for Tunick beats the number of people that turned out for Tunick's photo in Melbourne in 2001 when 4,500 stripped off.
Participants have been posing for more than an hour in a variety of positions from lying down, standing up and embracing cheek to cheek.
People of all shapes, sizes and ages are among the crowd including a pregnant woman who will be taken to hospital to be induced straight after the installation. The photograph subjects are upbeat with many cheering and clapping.
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"I want all couples to embrace and kiss, all friends to kiss and all strangers to do whatever they want," Tunick said as he directed the crowd.
People began arriving at the Opera House from 3am (AEDT) for the installation called Mardis Gras: The Base, which has been commissioned by the Sydney Gay and Lesbian Mardis Gras.
Student, Art Rush, 19, said he was thrilled to participate.
"I'll never get a chance to do this again, it's not worth being inhibited," Rush told AAP.
"It doesn't feel sexual, it just feels tribal, a gathering of humanity.
"I thought it would be all old people and nudists, but everyone here is great."
Even the Seven Network's Grant Denyer stripped off and joined the crowd, keeping his back to the camera as his naked behind was broadcast on the Sunrise breakfast television program.
