An "intense and fierce" fire that broke out inside a popcorn machine caused a cinema complex in Sydney's west to be evacuated overnight, authorities say.

About 300 people who were inside the Parramatta Event Cinemas had to leave when the fire started about 11:00pm.

Fire and Rescue Inspector Kernin Lambert said authorities arrived to find hundreds of people leaving the cinemas.

"Firefighters found a large-scale evacuation in progress. Approximately 300 persons were in the cinemas at the time the fire broke out," he said.

"The cinema complex was rapidly filling with smoke."

Inspector Lambert said the fierce fire began inside a popcorn machine.

"The fire itself involved a popcorn machine containing a fairly large quantity of popcorn which burnt quite intensely and quite fiercely," he said.

"The automatic sprinkler system operated and controlled the fire until firefighters were able to arrive and complete extinguishment."

He said the exact cause of the fire was still under investigation, but there were no suspicious circumstances.

It took firefighters about half an hour to extinguish the blaze.

They also undertook a large search to ensure no-one was still inside the smoke-filled cinema complex.

No-one was injured in the incident.