Police said the fire was deliberately lit.

Wentworthville Mall, in Dunmore Street, remains closed this morning as police and fire brigades inspect the damage.

Witnesses this morning said shoppers looked confused as they were trying to get inside the closed mall.

Fire crews were called Dunmore Street at 4.47pm yesterday, after the fire broke out in the Go-Lo discount store, inside the mall. It is believed the fire started inside the shop's loading dock.

Acting Superintendent Gordon Boath, the NSW Fire Brigades' zone commander for Parramatta, said the smoke was so thick crews had to use thermal imaging cameras to locate the blaze.

"The conditions were extremely hostile and thermal imaging cameras were used to locate the heat source,'' Acting Superintendent Boath said.

''Because there were very little openings for the smoke to escape, the conditions were very difficult and crews wore breathing apparatuses.

''The shop was just gutted by the fire and a number of businesses centered around the food court have been extensively damaged and other shops were affected by the heat and smoke.''

Acting Superintendent Boath said the fire was contained within three hours and was within the rear of the mall.

He said parts of the ceiling had collapsed.

Fire officers ad police remained at the scene this morning, continuing investigations into the blaze.

Police from Holroyd and Parramatta, 10 NSW Fire Brigades units - including 50 firefighters - and NSW Ambulance personnel responded to the emergency.

The shop had to be evacuated and no-one was injured in the incident.

Investigations are still continuing and anyone with information is asked to phone Merrylands Police Station on 9897 4899 or Crime Stoppers 1800 333 000.

Raw footage: Video of heavy smoke pouring from The Mall in Wentworthville in Sydney's west.

Click the link to watch the video footage...