A 24-year-old shooter tore through the waterfront construction site with a shotgun in the early morning, plunging the busy centre of New Zealand's largest city into lockdown.
Prime Minister Chris Hipkins said the gunman also died at the scene and there was no national security threat, so the marquee tournament could go ahead as planned.
Police believe the attack was not directly linked to the World Cup, nor was it politically or ideologically motivated.
But the gunman was known to police and said to have had a history of family violence and mental health issues.
Police said he had been subject to a home detention order but had an exemption to work at the site. He did not have a license to own a firearm.
Police Commissioner Andrew Coster said there had been "nothing to suggest he presented a higher-level risk".
Aucklanders had circled today's date as the start of a month-long festival of football that would showcase their city and country to the world.
Instead, they were woken by the sound of police sirens and helicopters thudding overhead.
Inside the building site, workers reportedly hid or attempted to barricade doors to save their lives.
Police Commissioner Coster said it was a "shocking and traumatic event for those people who came to work and found themselves in the middle of an armed emergency".
Viv Beck, who was having a coffee nearby at the time of the shooting, told AFP that the mood following the tragedy was "pretty sombre".
"It's been devastating. It must have been terrifying for the people involved".
She praised the police's quick response as "phenomenal. By all accounts, they did a good job."
"It was pretty reassuring," added Beck, head of a local business association.
At least three national soccer teams are staying nearby, including the reigning champion United States squad, which said all its players and staff were accounted for and safe.
Norway's team hotel was a few hundred metres from where the shooting unfolded.
Captain Maren Mjelde said players were woken up by a helicopter and "a large number of emergency vehicles".
Her team later faced hosts New Zealand after an opening ceremony and a minute's silence in honour of the victims of the attack.
Mass shootings are rare in New Zealand and sweeping gun laws were introduced after the 2019 Christchurch mosque massacre, in which 51 Muslim worshippers were killed and another 40 wounded.
Prime Minister Hipkins offered condolences to family and friends of the victims, saying "the whole nation is mourning with you".
The New Zealand premier promised a full investigation to see if "any red flags could have identified something earlier" to prevent the tragedy.
Hipkins has insisted the World Cup, which is being co-hosted by New Zealand and Australia, would go ahead.
The "assessment from officials is that there is no national security risk. There is no change to New Zealand's national security threat level," he added.
"The government has spoken to FIFA organisers and the tournament will proceed," Hipkins said. "New Zealanders' safety, and the safety of our visitors, are our first priorities."
Coster said armed police had quickly tracked down the gunman to a lift shaft at the site, where he had barricaded himself.
"The offender fired at police, injuring an officer. Shots were exchanged and the offender was later found deceased," Coster added.
"Tragically, police located two members of the public deceased on the lower levels of the building site."
A police officer was among a half dozen people treated in hospital. He was in a stable condition, according to officers.
Ahead of the World Cup kick-off game, Coster reassured match-goers and the general public that they could travel in safety to the city centre and Eden Park stadium.
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