The long queue for public housing could have been shortened if the Housing Department had made better use of several thousand flats left unused for various reasons, the watchdog says.
The Audit Commission report said the government had failed to fulfil its pledge of a three-year waiting list as almost a third of the 116,000 general applicants had waited longer than that.
And while the Housing Department claimed a vacancy rate of 0.6 per cent - well below its pledge of 1.5 per cent - the report said the rate could in fact be as high as 1.7 per cent.
It said the department took into account only 4,137 units it deemed "lettable".
"The 4,370 'unlettable' flats and 3,964 'under offer' flats, which were also not occupied by any tenants, were not included as vacant flats in calculating the vacancy rate," the report said.
"Unlettable" flats are reserved for various reasons, including relocation of tenants affected by redevelopment and those under conversion or being repaired.
But the commission found that 618 out of 4,370 "unlettable" flats had been vacant for 10 years or more, with 765 being vacant for from five to 10 years.
Meanwhile, in site visits to three housing estates, the audit found that 43 "under offer" flats - those offered to tenants and supposed to be occupied within two weeks from the issue of an offer letter - had been vacant for more than three months, with 14 for more than a year.
The refurbishment period of some vacant flats was also found to exceed the target time of 44 days, ranging from five months to more than three years, the report said.
The commission called for the Director of Housing to speed up the letting of "under offer" flats that had been vacant too long and to indicate how long it was expected the "unlettable" flats would be reserved so as to make better use of them.
Legislative Council housing panel chairman Wong Kwok-hing said it was unreasonable for Housing Department not to take the unoccupied flats into account in calculating the vacancy rate.
"It was not a small amount and leaving some of the flats vacant for 10 years is unacceptable," he said.
Housing minister Anthony Cheung Bing-leung promised to follow up the recommendations.