Auckland Council is buying new headquarters more than four times the size of its civic administration offices at a cost of $104 million.
The 35-level building on Albert Street currently houses the ASB Bank.
The move will allow the council to eventually vacate five other downtown premises.
Chief executive Doug McKay says the council will vacate its civic building at the end of next year, and two major leased premises a year later.
He says initially the shift will save $2.7 million a year, rising to $5 million a year, after a couple of decades.
"We'll borrow to buy and our cost of borrowing will be a lot less in the short term than the return we get off the leases from the ASB and the third party tenants, so that gives us a bit of upside in the first year or so".
The new building can house 2400 of the council's 6000 staff.
Mr McKay says the council hasn't yet looked how, or whether, the new building might connect to the adjacent Aotea Station on the proposed downtown rail tunnel.
Future uncertain for civic building
Auckland Council says the fate of its existing civic administration building uncertain.
Mr McKay says the 46 year old civic administration building has asbestos problems and would cost $93 million to refurbish.
He says options to be weighed up in coming months include the council retaining the building, selling it, or demolishing it and reusing the land.