The 22-year-old was forced to stop the show as staff at the Basement Theatre called the sick man an ambulance, cleaned up the mess and opened the doors for those that wanted to leave.
"He wasn't drunk or anything. He was just a guy who wanted to come out for the night and had some sort of stomach flu. I hope he didn't pass it around," she said.
Around a third of the audience chose to leave, but organisers asked Bracewell if she wanted to continue the show for those that stayed.
"I asked the crowd that was left and they were like, 'Yeah'. It smelt a bit. They were legitimately cleaning it (as I performed) ... All the chairs that weren't covered in stuff they moved to the front, so the audience were pretty much on the stage with me, fully lit.
"I had to press on but every so often just acknowledge the fact that this is very, very weird."
The Billy T Award-nominated comic has been performing stand-up comedy for more than three years and has received rave reviews for her new show Melodrama, which is running as part of the International Comedy Festival.
But the Hauraki radio DJ said it was the first time someone had vomited during a show.
"You expect drunk people to yell something out. I had a show where a woman was feeling quite faint - but nothing to this extent," she said.
"It's certainly not what I was expecting or what was planned for the show. It was a wild time."
Bracewell's show Melodrama runs at The Basement until Saturday night.
Herald critic Gary Dooley called her a "fully-fledged star, saying her show was "packed with laugh-out-loud moments galore and endearing storytelling, delivered by a comedian fully in control of her craft and her audience".
Read more: Comedy review: Melanie Bracewell, Melodrama