The company owns the land and will also own the 9500 sq m building, of which 6000 sq m was office space and about 75 carparks below street level.
TrustPower has signed a 15-year lease with a 15-year right of renewal, Mr Hawksworth said.
The building is big enough to house the 350 existing staff based in Truman Lane and the Westpac building in Devonport Rd, and allow extra space for growth.
Mr Cooney's company CBC Construction which was responsible for the $30m ANZ building on the corner of Cameron Rd and Elizabeth St will undertake the build, which was expected to be completed in 102 weeks or February or March 2016.
Two to three other professional "business-type" tenants would be sought, he said.
Dwayne Roper said the final phase of the demolition of the Durham St building would start on Monday and was expected to take three-four weeks, and earthworks commencing in June.
Mr Cooney and Mr Roper said this was significant exciting development, which they hoped would generate added confidence to encourage other developers to invest in the city.
"Not only does it mean $25m will be injected into the local economy, but it also represents the creation of lots of local jobs, with up to 100 people working on the project at various times. It's a truly locally designed and driven project," said Mr Cooney.
He said the design of the building would be unique as it would be an open plan development with an atrium, and have some special features because of TrustPower's infrastructural requirements.
Mr Hawksworth said TrustPower had spent 12-18 months looking for a suitable location before settling on the site and the new building represented an exciting phase in TrustPower's major expansion plans both within New Zealand and Australia.
Mr Crosby said the development was another huge boost for Tauranga's downtown economy.
"It's exciting that TrustPower will be basing itself in the city. This development will obviously add some extra vitality to the central city and it's great to see local contractors and local developers involved in this project."
Council's role in the new venture is to assist with making sufficient leased parking spaces available in and around the new building for a period of up to five years, Mr Crosby said.
By the numbers
Price tag $25m.
Three-storey 9500 sq m building, up to 75 carparks below street level.
Demolition of the old building expected to finish in three to four weeks.
New build earthworks expected to start in June.
Building ready to occupy early 2016.
Main lease tenant TrustPower will occupy 2.5 floors.
Several other professional business tenants to be sought.