Colliers' Who Will Rule Our CBDs? research report says over six months to January at least 87,795 square metres was added to the office market while net absorption was just 40,000 square metres.

Vacancy rates rose to 9.1 per cent over the same period but despite the uptick and extra 50,000 sq m of space, they would "not blow out to double figures in 2015".

"We still expect that incentives for Melbourne CBD office space will remain at current levels," the report said, as few owners were willing to offer above 35 per cent given that internal rates of return for institutional grade assets were already stretched.

Major law firms currently looking for prime grade space include King & Wood Mallesons, Clayton Utz, Minter Ellison and Norton Rose.

Activity was picking up for secondary grade office space.

"Over 200,000 sq m of secondary grade office space is occupied by tenants whose leases are expiring over the next three years," the report said.

Ownership of CBD offices was becoming more concentrated.

Australian institutions still dominate but offshore ownership had increased from 9 per cent in 2009 to 14 per cent.

The largest reductions were in private and government ownership, a trend expected to continue.