The Buildings Department on Monday revealed that concrete that plunged into a busy Mong Kok street on Sunday came from an illegally built canopy on the 16th floor of a building that was subject to a mandatory repair notice back in 2014.

Pieces of concrete plunged into Mong Kok Road on Sunday afternoon from the Po On Building, hitting the truck and injuring the driver, who went to hospital with back pain. Police arrested two men on suspicion of allowing an object to fall from height.

The Buildings Department said its officers went to inspect the 57-year-old building on Sunday afternoon and found that the canopy fell from an external wall on the 16th floor of the building. It said other other canopies and projections on the wall had also come loose.

"No obvious danger to the overall building structure was noted," it said in a statement. "With the assistance from the Fire Services Department, the loose items on the external walls were removed."

The department said the building's owners corporation was issued with a repair notice under the Mandatory Building Inspection Scheme in 2014 in relation to problems with common areas.

It added that the corporation had applied for a subsidy under an Urban Renewal Authority scheme, which had been approved two years ago. The department said it had received a report from an inspector appointed by the corporation in March this year, and work is under way to tender for a contractor.

The department continued: "Although the concerned building has been inspected, the statutory notice ... will not be considered as complied with until the repair works have been completed."

The department said it "will also consider taking enforcement action if necessary so as to ensure due compliance with the statutory notice and the safety of the occupants and the public".