The storeroom of Esplanade post office was gutted on Friday afternoon.
Files, parcels and sacks filled with post, journals and books were destroyed by the blaze and the water sprayed by firemen to douse the flames.
The ground floor of the post office was, however, unaffected.
Firemen used six engines and took one hour to put out the fire. They said a short-circuit in an air-conditioner could have caused the fire.
Fire officers pointed out that the building lacked fire-fighting mechanisms.
Mihit Sadhukhan, a hawker in Esplanade East where the post office is situated, saw thick smoke billowing out of a second-floor window.
“Within seconds the amount of smoke wafting out of the window increased so much that it spread panic among the hawkers. The post office workers were immediately alerted ,” said Sadhukhan.
The 100-odd workers of the post office evacuated the building.
The 2,000sq feet storeroom was used by the post office authorities to store unclaimed parcels and undelivered letters, magazines and journals.