The blaze which started in an office at Betta Electrical on Gardeners Rd, Kingsford is currently being investigated by the NSW Fire Brigade.
Fire and Rescue NSW Superintendent Paul Johnstone said the quick response of fire fighters stopped the fire spreading to buildings which included a church and petrol station nearby.
The store was so badly damaged that the Fire Brigade ordered that it be demolished immediately.
It is not known whether the store which employed more than 25 people will be reopened.
Police from Eastern Beaches Local Area Command established a crime scene at the location.
Fire and police investigators are examine the cause of the fire and have urged anyone with information about suspicious or unusual activity in the area to come forward.