There were a couple of larger companies looking for space pre GFC but decided to sign on in their existing buildings when the GFC hit. Origin Energy was one. Some of these companies and others will be reviewing their leases again soon. A large government department could also be interested. I'm sure a number of companies are aware that premium office space could become rare in Adelaide if the mining boom really does occur soon. They need to plan for this and lock in good deals in advance. Plus the attraction of 'green' office space.
Obviously CC4 has to get approval, then pre-commitment before starting. Aspen would be speaking to potential tenants well in advance.
CC8 took 2+ years after approval to find a tenant, thanks largely to the GFC.
This part of the city is highly desirable for large companies. It's an excellent location with everything on offer.
Don't forget the completion of CC8 allows Australia Post to move out of the GPO, has removed the electricity substation issue and effectively frees up the remainder of the site for development.
Also Aspen also has a great track record with City Central and can clearly show prospective tenants what they can offer with tours of the completed buildings. Unlike some of the other developers in town developing one off buildings, some with no prior examples of other projects.