"Then towards the end, suddenly one of them let out a scream. My sister pointed out, 'Oh my god, look! Something's happened to her wire'. She looked like she was at an angle and that one of the wires may have twisted or broken, it went on like this for about 30 seconds."
She told Fairfax the singers stopped singing, which then prompted the actors below to look up and run towards the area where Leah was dangling.
"She looked petrified because she was quite high up, the bus was turning on a stile and they think maybe she was too low down and it hit her."
The unplanned on-stage drama brought the whole show to halt as they sorted out the problem.
O'Sullivan said the music stopped and the curtain was drawn before a voice advised the audience something had gone wrong.
"You could hear the whole audience talking about it because it was a pretty full house, we were sitting there for about five minutes wondering if it was going to come back on."
She said the routine eventually started again without Leah.