New Zealander Natasha Nicholls was one of those trapped in a locked-down building near the cafe.
Ms Nicholls, a receptionist at PTW Architects, told Checkpoint she alerted people in the firm to the siege after being warned by her boyfriend.
She said her heart went out to staff at the Lindt Chocolat Cafe.
Young girls worked there, and they must be terrified about what was happening, she said.
"It's a little bit more real because I've been in that building. The windows are quite tinted, so they must be absolutely frightened.
"When you are sitting there, doing your everyday duty and then that happens. They must be absolutely traumatised."
Ms Nicholls said it was a peaceful cafe and they would have had a lot of customers going through for Christmas.
Ms Nicholls had since been allowed to leave the building.