And despite state projections this year for job growth that leads to greater office occupancy, real estate brokers said the weak leasing market will likely last through the rest of the year.

"It seems like maybe we've hit the peak of vacancy, but it hasn't turned down yet," said Jamie Brown, president of Hawaii Commercial Real Estate LLC.

Around downtown, the equivalent of a 12-story office building emptied during the past six months, expanding total available space by 131,000 square feet to 1.1 million square feet, or the rough equivalent of a 100-floor tower, according to the report by local commercial real estate firm Colliers Monroe Friedlander.

The bulk of the loss took place in the most modern "Class A" buildings, which accounted for about 10 of the 12 newly vacant floor equivalents.

Another local commercial real estate firm, Grubb & Ellis/CBI Inc., said the downtown office tenant exodus occurred mostly in the first three months of the year, and pushed the vacancy rate to 15.3 percent, above the U.S. average of 14.6 percent.

Among the companies that reduced or put their office space up for sublease, according to Colliers and Grubb & Ellis/CBI, were Verizon Communications, Genesys Telecommunications, Spirent Corp., GE Capital, Carter & Burgess, Gas Co. and Tesoro Petroleum.

Downsizing and consolidation by companies, especially telecommunications and Internet-related firms, continue to build on a vacancy surge in Honolulu that started after Sept. 11, 2001, and hasn't let up.

According to Grubb & Ellis/CBI, 32 downsizings and nine company closings since 2001 drained office space on O'ahu, nearly all of it in downtown Honolulu.

Colliers said nearly three years of rising downtown office tenant losses put the vacancy rate at 14 percent at the end of June, and have increased pressure on landlords to offer concessions such as temporary free rent, moving expenses and renovation allowances.

Grubb & Ellis/CBI, which tracks the market slightly differently, puts the downtown vacancy rate at 16 percent through June.

Rental rates, however, have held firm over the past three years, Colliers said. On O'ahu, the average asking rent per month over the past six months dropped by 1 cent, to $2.12 per square foot.

"Landlords are often hesitant to change their asking rents even in down market cycles," Mike Hamasu, Colliers research and consulting director, said in the report.

Grubb & Ellis/CBI also said it expects rental rates to remain stable.

Still, with growing leverage to obtain concessions, businesses in the market for office space are going to see more attractive opportunities, brokers said.

Colliers said it is aware of companies needing about 260,000 square feet of space that likely will be leased later this year.

Much of the demand is from businesses linked to Hawai'i's hot residential real estate industry, such as mortgage and escrow services. But given the time it takes to fill office space, Colliers projects the market will still lose more tenants than it gains this year, with 45,000 square feet of additional empty space and an islandwide vacancy rate of 14 percent at the end of the year.

Grubb & Ellis/CBI projects up to 75,000 square feet of additional empty office space this year, pushing O'ahu's vacancy rate above 15 percent.