Police say a fire in a home decorator store in Sydney's northwest was deliberately lit and are urging witnesses who may have seen suspicious activity to contact them.

Emergency services were called to Victoria Avenue in Castle Hill at about 11.30am (AEDT) on Sunday after a customer saw smoke and flames in the floor rug section of the store.

As staff evacuated the shop another employee put the blaze out with a fire extinguisher, confining damage to a stand of rugs.

Police believe the fire was deliberately lit and want shoppers who were evacuated from the store to contact them if they noticed someone acting suspiciously at the time of the fire.