The trust is selling the first half for $47.5 million to Auckland City Holdings, a company associated with the Brisbane-based Bloomberg Incorporation.
A 50% share of the complex was valued at $48.3 million at 31 March.
Chief executive of the trust's manager Chris Gudgeon says the sale is consistent with the strategy of recycling capital out of mature assets.
Kiwi Property Management will continue to manage the building which is currently 79% occupied.
The other 50% of the complex will be sold in either March or September of the years leading up to March 2017, at the option of the purchaser, at which point it must buy the stake.