Mr Purches confirmed Trustpower had signed a memorandum of understanding with local developer Peter Cooney and Zariba Holdings, which owned the Durham Street site.
"If everything does go according to plan the objective is to build a purpose-built building on the site which would probably be three storeys, big enough to house 450 people and be ready for occupation within two years, Mr Purches said.
The proposed new headquarters would not only house the 300 staff occupying the company's Te Maunga-based headquarters but also the 50 people housed on the top of Westpac building, plus have sufficient space to allow for growth.
"While we are pretty close to reaching a deal there are still quite a few details which need to be finalised, and we also need to have the paperwork looked over by our lawyers," he said.
Mr Purches would not be drawn on how expensive the building project was likely to be.
He said the new building proposal was all part of Trustpower's major expansion plans in New Zealand and South Australia.
Trustpower, which bought Energy Direct in mid-2013, was not only exploring a number of other opportunities to expand its retail business in New Zealand, but was at the moment spending A$450 million ($488.2 million) to expand one of its two wind farms located in South Australia, he said.
"Lots of people in Tauranga think Trustpower is just a little power company but we are also a significant gas supplier, and a stand-alone internet service provider.
"That means any new building needs to be a lot more sophisticated than the average office building."
Developer Mr Cooney and Zariba Holdings director Tom Roper referred the Bay of Plenty Times inquiries to Trustpower.