It is understood the facility has the capacity to eventually house up to 200 staff.
Danny Blair of Colliers Hawke's Bay said it took 10 months of negotiations to broker the deal to bring Kiwibank to the CBD site, which has been vacant for about two years.
Mr Blair said it was Hawke's Bay's largest office leasing deal and "will greatly impact the area, both in its economic development and overall reputation as a business destination".
Adjacent land, on the site of what used to be the County Club, will be converted into car parking for Kiwibank, he said.
Colliers was in negotiations with other "large occupiers" interested in other nearby vacant Hastings CBD sites, Mr Blair said. "We've got a fair bit of activity on the old Warehouse and old Mitre 10 Mega store on St Aubyn St," he said
"We've got quite a bit of interest in those as well and we're expecting some results there this year.
"Mitre 10 Mega could be this month. The big holes in Hastings are starting to fill up."
For commercial reasons, Mr Blair said he could not name the companies Colliers was negotiating with, but he said Kiwibank's move would have a positive impact on the city's real estate market.
"The [Kiwibank] relocation proposed to occur in October this year will greatly impact the commercial real estate industry within Hastings in both sales and leasing," he said.
"The new office addition is seen to bring added diversity to the region and increases the incentive for other businesses to relocate to the Bay."
Hastings Mayor Lawrence Yule said Hastings District Council staff had worked closely with Kiwibank and the property owner, HBM Ventures, to make the move possible.
"I believe the business they will bring to Hastings will be a major boost to the city centre and the district," he said.
"The establishment of the business will not only provide a much-needed boost to the job market in Hastings but will bring further vibrancy to the CBD."
Susan McDade, city centre manager for the Hastings City Business Association, described the Kiwibank deal as "the biggest change agent that Hastings has seen in probably the last five years".
She said it amounted to bringing a new industry into the region, which would have a major impact on the local economy.
"We're really excited. Anytime that you bring over 100 jobs to a small provincial centre like ours it's going to benefit the whole region.
"It's not just a Hastings win, it's a Hawke's Bay win," Ms McDade said.
Factors that contributed to successfully selling Hastings as a site for business relocations included the low cost of relocating, cheap leasing costs, the availability of earthquake-compliant buildings, the availability of a local workforce and support offered by the council, including rates relief, she said.
"If we get a couple more like that it changes the economy of a town.
"You start to see the effect of businesses coming in.
"I think we are in a pretty good position moving forward for this next year."
Mr Blair said Colliers was actively pitching the option to relocate to Hawke's Bay to specific businesses.
"We're going out and targeting clients, trying to get them here and giving them good reasons why they should be operating here.
"Obviously it's the cost, life-style, all that."
Speaking on behalf of HBM Ventures, Hastings property developer and consultant Terry May said the Kiwibank building was expected to be ready for occupation in early December.