Enough office space to house about 5000 employees has come up for sub-lease, as debt-stricken financial firms try to figure out ways to pay landlords for some of Sydney's most prestigious office towers.

In just weeks, the amount of sub-lease space available in the city's finance district has rocketed to 70,000 square metres, from 43,000sqm in October.

The amount available could grow to more than 80,000sqm -- about the same amount of floor space as in Sydney's 44-storey Grosvenor Place -- if investment bank Babcock & Brown fails. The bank, which has $3.1 billion worth of debt to renegotiate, has 10 floors in the exclusive Chifley Tower. So far, only two floors are on the sub-lease market. Last month, 11 financial sector tenants were offering up space for sub-lease.

The 70,000sqm of office area available is understood to include 7000sqm worth being offered by Insurance Australia Group at IAG House on George Street and four floors held in Gateway at Macquarie Place by the failed finance firm, Allco Finance Group.

The Australian