Grade-A office rents have fallen 9 per cent since February, according to Colliers Jardine.
Central and Admiralty rents recorded a faster rate of decline - more than 10 per cent - due to the contraction of the rental premium against other districts.
Also, decentralised grade-A office buildings were slower growing than the central business district and so suffered less in the falls, Colliers Jardine said.
It was likely the trend would continue for 12 months, especially before the full completion of the two major Central office developments - 11 Chater Road and Two International Finance Centre - providing a total gross floor area of more than 2.4 million square feet.
Corporate downsizings in the first half of this year were triggered by the dip in the United States economy.
A lack of new supply meant activity had been sluggish in the primary market and, coupled with a sharp fall in new demand, the office leasing market had seen a dearth of leasing transactions.
With a lack of new demand, new space would be filled principally by relocations, further increasing secondary-market vacancies, Colliers Jardine said.