The building, on 202 Parnell Rd had a valuation of $6.1 million and sits on 1813 square metres of land.
REINZ will remain on as a tenant in the building until at least April next year.
Bayleys real estate agent Cameron Melhuish said the property attracted a wide range of potential buyers - some wanting the building as an owner-occupier space, with others looking at mixed use commercial and residential development opportunities.
"The healthy interest received in this property shows that there is on-going demand for well positioned commercial real estate in the Auckland city fringe - especially land and buildings with mixed-use potential."
The sale was open to overseas buyers through an international sale process.
REINZ chief executive Helen O'Sullivan said the company was looking for a smaller head office as its staff numbers had done from between 40 and 50 to 16 in the past three years.
It was staying flexible on the suburb but were looking for a modern premises.
The company had abandoned 2011 plans to expand the building. They had upset Monsignor Kevin Hackett from the neighbouring Catholic St John the Baptist Church, who feared shading, increased traffic movement and the potential to destabilise the sloping site.
Other plans granted in 2009 allow for a four-storey residential development behind the building, including a pair of two-storey dwellings in the middle levels and a penthouse on the top floor.