Police said the fire was deliberately lit.

Wentworthville Mall, in Dunmore Street, remained closed on Tuesday morning as police and fire brigades inspected the damage.

Fire crews were called to Dunmore Street at 4.47pm on Monday, after the fire broke out in the Go-Lo discount store inside the mall. It is believed the fire started in 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 find the blaze.

"Because there were very few openings for the smoke to escape, the conditions were very difficult and crews wore breathing apparatus,'' Superintendent Boath said.

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

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

He said parts of the ceiling had collapsed.

Fire officers and police remained at the scene throughout Monday night and Tuesday 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 continuing and anyone with information should phone Merrylands police station, 9897 4899, or Crime Stoppers, 1800 333 000.