It is 8.30am on Sunday in the middle of Sydney city and there are bodies strewn across the ground, cars flattened with concrete blocks and a train derailed.

Elite units from NSW's emergency services, including the Fire and Rescue urban search and rescue team, are trying to swiftly move people out of harm's way.

Immense resources were thrown at the staged scenario on Sunday morning to test how the city would deal with an explosion.

It was the first training exercise of its kind for the city.

"It is really unprecedented, the amount of resources and time that has gone into planning this event," NSW Police Assistant Commissioner Michael Fuller told reporters.

The scenario was centred on a gas explosion, which prompted a building collapse on the corner of Macquarie St and St James Rd, not far from Parliament House.

The hypothetical blast also caused a train to derail near St James Station.

Outside the Law Courts building, dummies were arranged on the ground while cars topped with concrete blocks - and plastic limbs hanging out of smashed windows - were on the street.

A structure across the road mirrored a building collapse with hundreds of volunteers role playing casualties and evacuees as part of the exercise.

About 500 personnel gave up their Sunday morning to participate in the training event, which is part of a wider review of the city's evacuation plan.

"This type of incident will test the arrangement of emergency services in terms of evacuations, treating high levels of patients and how agencies work together to ensure the highest level of safety for the people of Sydney," Mr Fuller said.

Police stressed the exercise wasn't linked to last year's Lindt Cafe siege.

While Sunday's exercise was emergency based, a criminal event, like a bomb explosion, would test similar arrangements.

Mr Fuller said the exercise would become a yearly event.

The exercise has closed some roads in the city's east but is expected to wrap up at midday.