Office space on O'ahu got more expensive and less available during the third quarter, continuing the market's tightening trend, according to new industry reports.

Businesses filled the equivalent of a 13-story building, or 130,000 square feet, in the July-to-September period, according to commercial real estate firm Colliers Monroe Friedlander.

The leasing activity cut vacant office space to about 1 million square feet, or 6.85 percent of total office space on O'ahu. That was down from 7.69 percent at mid-year. Average rent per square foot rose to $2.76 in the quarter, up from $2.45 at mid-year.

Another commercial real estate firm, PM Realty Group, said the downtown Honolulu office market could see more dramatic rent increases if the current rate of leasing continues for another 12 to 18 months.

Colliers said inflation, low unemployment and a projected decline in home construction may slow the rate of office leasing.

No office high-rise construction is under way, meaning no large inventory will likely be added to the market for at least the next two years. However, as rents rise, that makes office tower construction more feasible.

Reach Andrew Gomes at agomes@honoluluadvertiser.com.