The fire took hold within minutes according to firefighters who arrived at Carpet Choice store to find flames leaping out of the roof cavity about 6.45pm.

About 50 firefighters attended the scene, closing a 600m section of the Princes Hwy between Cabbage Tree Lane and Mt Ousley Rd.

Rolls of carpet and rubber were ablaze, sending toxic fumes into the air, while outside firefighters doused the blaze from a cherry picker. The fire was contained by 7pm.

Firefighters continued to monitor the store's rear storage area which smouldered long after the flames had been extinguished.

Police detectives established a crime scene around the blaze and interviewed witnesses.

Acting Inspector Nick Skomarow said the cause had not been identified but that investigators would comb through the building once the fire was extinguished.

"It is a crime scene until the fire is out and our crime scene investigators carry out their investigation," he said.

"The fire was contained to the one building - the other buildings might have some smoke damage."

NSW Fire Brigade Superintendent Malcolm Connellan said the carpets and rubber underlay contributed to the intensity of the blaze.

"It is a flammable material and is usually stacked, rolled up pretty high at the back of the building ... it smoulders and smoulders'," he said.

"We will wait until we get it under control and investigators come in to determine source and origin."