The excavation of the rest of the site would be staggered so that the construction of foundations just kept ahead of excavations.
"It will be all action from July onwards," Mr Roper said.
The 10,000sq m Trustpower building will be a similar size to the $30 million ANZ building developed by Mr Cooney on the corner of Cameron Rd and Elizabeth St.
Mr Roper said when Trustpower's new head office was finished in March 2016 it would house 400 staff who would shift from Te Maunga and the Westpac building in Devonport Rd.
"Having this number of extra people in town will be a positive."
The finished building will have a basement carpark at the Grey St level for up to 75 cars. The three levels above Durham St will feature a big atrium.
Mr Roper said building consent for the first stage foundations was lodged with the council last Friday.