A third man, understood to have been crouching beside the driver's door, also died.
"It was just panic everywhere," a witness said.
A concrete slab that had been hanging off a crane crashed to the ground, and a worker who was harnessed to the frame of the building was left dangling off a gantry.
The workers were from Hawkins Construction and subcontractors Vuksich & Borich Civil Engineering.
Debris was flying in every direction and the onsite office was in ruins - to such an extent the foreman had to crawl free.
A workman told the Herald at the scene of the desperate search for Brendan, a labourer aged in his 20s.
He said they looked for him in his car, parked close to the spot where he was last seen, and called out to him. They feared he had been blown onto a reserve that was littered with debris torn from houses.
The reality, though, was far worse.
"Someone heard that we were looking for him and said they had seen him. I think it was a passenger who had been in the truck ...
"He described him [Brendan], his hair, so we knew it was him."
The news was devastating. "It's pretty hard when you have to tell [the boss] that we've lost one of the boys."
Other workers said they had no other choice but to cling to a large truck and "hold on for dear life".
"I've never seen anything like it," one said. The concrete slabs must have weighed 20 tonnes, he said, but they were being picked up "like they were small rocks".
As they watched the body recovery, the workers spoke of receiving no warning bad weather was on its way, even though weather alerts had been issued for more than an hour.
The recovery of the bodies was hampered by heavy rain and the threat of another tornado.
Warnings to take cover were broadcast from a Fire Service vehicle, with residents, emergency services and staff from Whenuapai Air Base taking shelter in shattered homes.
Truck driver Kevin Johnson said the area had been "flattened", with trees and roofs down everywhere.
Shocked residents were barely able to believe what had just happened.
Concrete cutters were used to try to break the slab up and a crane was brought in to try to lift it.
Hearses arrived at the scene just after 6pm.
Hawkins Construction executive general manager Dan Ashby said the company's immediate focus was on supporting the affected workers' families, and its staff and subcontractors on site.
A staff member at Vuksich & Borich said a company worker was one of those thought to have died.
Long fine spell from tomorrow
The worst is over and clear skies are coming for the weekend, forecasters say.
A strong westerly was making its way northwest over the country last night and is expected to shift to a westerly path early today, bringing some showers to most of the country.
WeatherWatch chief analyst Philip Duncan said there would be clear skies tomorrow. "Most models I've looked at have shown that we're in for pretty much 10 days of dry weather for most parts of the country."
Mr Duncan said the chance of more tornadoes was extremely slim, "although it is still a bit humid, which is why we're saying there is still a small chance of one".
The MetService said showers would become more frequent and heavier today in the west of the North Island before starting to clear tomorrow.
