Witnesses saw the vehicle being driven erratically before it hit the metal structure, without decelerating. Earlier reports that Mr Willison died while reversing his ute were incorrect.
The Herald understands Mr Willison worked for New Zealand Steel Mining, which mines the site near the isolated settlement.
It is believed colleagues tried in vain to save him using a defibrillator.
His brother, Charles Willison, is the acting manager for Maori Incorporation Taharoa C Block, the Maori landowners of the site at the tiny Waikato west coast village, near Kawhia.
Te Kuiti police sub-area manager Senior Sergeant Rob Van Kalken said it would be for the coroner to determine the cause of Mr Willison's death.
"The Waikato serious crash unit is investigating the incident but because the death was at a work site and not on a road it is not being treated as a fatal road crash," Mr Van Kalken said.
The Department of Labour had also been advised because the death occurred at a work site.
"The deceased was the only person in the vehicle at the time and the coroner will determine the cause of his death."
Mr Van Kalken said Mr Willison came from a large whanau and police expected to notify all next-of-kin by last night.
It was not known if he was married or had children.
The Taharoa mine covers 1300ha, segmented into three regions, and is leased by the Maori landowners.
Mining began there in 1972 and since then New Zealand Steel Mining has supplied Asian customers.
The company's products include Coloursteel roofing, Zincalume steel and Galvsteel.
