Dozens of police vehicles, a helicopter and several ambulances are attending the lower Queen Street site on Thursday as the city prepares to host the opening match of the Women's World Cup.
Media reported the shooter was a 24-year-old man who worked at the site, and who had arrived at the building with a gun.
Police said he "moved through the building site and continued to discharge his firearm".
"Upon reaching the upper levels of the building, the male has contained himself within the elevator shaft," the police statement read.
"Further shots were fired from the male and he was located deceased a short time later."
Police said the shooting is not a national security risk.
"What has unfolded is understandably alarming and we are reassuring the public that this incident has been contained and is an isolated incident," the statement said.
St John Ambulance have confirmed six injuries, three serious and three moderate.
Prime Minister Chris Hipkins has changed his plans to travel to Hamilton for a climate change announcement and is expected to brief Kiwis on the attack this morning.
Locals reported hearing gunshots before 8am, with TV outlets broadcasting workers at the building site hiding behind packs of pre-mix cement.
Ferry services, which use a nearby terminal to operate, have been cancelled and locals asked to stay away.
The incident has taken place on the first day of World Cup play, with NZ set to play Norway in the tournament's opening match at Eden Park, five kilometres south.
Norway are staying at a hotel in close proximity to the shooting site.
The FIFA Fan Festival is also nearby, where former NZ international Maia Jackson was spending her morning.
"It's pretty scary actually ... there's lots of security and lots of uncertainty," she told the NZ Herald.
Ms Jackson said Norway remained in full lockdown.
Mayor Wayne Brown told TVNZ the attack was "dreadful".
"It couldn't have come come at a worse time given the world has its eyes on us now with the FIFA soccer thing," he said.
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