The Director of Audit's report found that 16 public heritage sites either were either not being maintained or had not be used for periods of up to 20 years.
Ten of these buildings are "unallocated" to any bureau or department, which means the Architectural Services Department only provides emergency repairs to address immediate defects or danger, according to the report issued yesterday.
"Given the historic values of the sites, the audit considers it unsatisfactory that 10 unallocated government-graded buildings had been left unattended," the report said.
These sites include two with grade one ratings, the 1887 Old Dairy Farm Senior Staff Quarters in Pok Fu Lam, the oldest remaining building of the dairy company, and an observation post at Tiu Keng Leng, built before 1898 for stationing Chinese troops.
The audit also found the Old House at Wong Chuk Hang San Wai, which was intended for use as a folk museum, had lain idle from 1992 to 2012 and only opened to the public on weekends and holidays this year.
The Development Bureau needed to take measures to ensure all public graded sites were properly maintained and gainfully used, the Director of Audit added.
As for private historic buildings, the audit revealed that six blocks in the New Territories had been demolished without obtaining approval from the government.
The sites were unnamed, but government sources said the demolition of five sites was made known by the Antiquities and Monuments Office earlier. The remaining site is Tseuk Yuen, a private home in Yuen Long built by a rich villager in the 1930s. The owner demolished the Chinese-style house and a pavilion in 2011 without Buildings Department consent. Only the entrance gateway remains.
Antiquities Advisory Board chairman Andrew Lam Siu-lo said the issue of private heritage sites would be considered in a new heritage policy review.
"There is no law that requires owners of certain New Territories buildings to obtain government approval for demolition," he said. "The review will see whether the law has to be changed to prevent this happening."
The board will have a closed-door meeting with a consultant this Saturday regarding the feasibility of setting up a heritage trust.