Authorities are undertaking Sydney's biggest-ever emergency simulation exercise, with over 1000 people testing what would happen if a plan crashed in the CBD.
The test, that started at 9am on Sunday, is being carried out at Barangaroo to test how ready the city is to deal with a large-scale emergency.
More than 700 personnel are involved in the exercise, including 300 volunteers role-playing family and friends of those involved in the incident scenario.
The set-up involves a plane that sustains damage in take-off from Sydney Airport and ends up crashing at Barangaroo.
A number of agencies are involved in the test, which is scheduled to conclude at 2pm.
Assistant Police Commissioner Michael Fuller said the test was the biggest of its type ever seen in Sydney.
He said it was part of the city preparing for a worst-case emergency scenario, with many casualties.
"We continually review our emergency plans and today's exercise is part of the ongoing review process and provides police and emergency service organisations with the guidance required in a range of events, including ones where mass casualties are involved," he said.
Fuller said people in the Sydney CBD today shouldn't be concerned by the dramatic test.
"Today's exercise is also about engaging with the public and displaying the co-ordinated emergency arrangements to demonstrate that if there was ever an incident of this magnitude, we are ready to respond to minimise the loss of life and property," he said.
"Having plans is one thing, but testing those arrangements with all agencies involved in a realistic setting is imperative."
Sydney Airport CEO Kerrie Mather said the test was one of many dramatisations the airport regularly did to test its emergency response capabilities.
"It's our priority to deliver the highest levels of safety and security for our passengers, stakeholders and staff," Mather said.