Sydney's Central Station has reopened all platforms following a fire on the concourse level that prompted an evacuation.

Emergency services were called in just after 6:00pm after a fire alarm went off within the fast food area.

The station was evacuated while 40 firefighters brought the blaze under control.

Superintendent Mark Riley said it was unclear what caused the fire.

"At this stage the firefighters and police forensics are conducting an investigation to determine the cause of the fire," he said.

"The fire's been contained to the area of origin of the restaurant and hasn't spread from the restaurant to any other adjoining properties."

Earlier, commuters shared photos on social media showing smoke billowing out of the century-old building.