Escalated violent crime is expected to be a problem for years to come, before the results of gun control measures will be seen, New Zealand's top cop says, following Auckland's construction site shootings.

On Thursday Matu Tangi Matua Reid, 24, walked onto a construction site in lower Queen Street, where he had been working, and opened fire with a pump action shotgun.

Three people died, including Reid. Six others were taken to hospital, including two police officers.

Police Commissioner Andrew Coster says that firearms controls introduced in recent years were expected to reduce the number of guns in circulation in the community, but it would take time.

"We've known for quite some time that our firearms environment is not where we need it to be. That was something that was well traversed post the 2019 attacks in Christchurch - hence the changes to gun legislation - the firearms register went live in the last couple of weeks," Coster says.

"Overseas experience suggests that it takes up to a decade or longer to really show the real benefit of that.

"In the meantime we have too many firearms circulating in the wrong hands and that creates a range of challenges for our frontline staff - so it will improve with time, but we need to keep focused on the [problems] that we have under way."

Reid did not have a firearms licence. He had recently been sentenced on family violence charges and was serving a five-month sentence on home detention.

Coster says there was no suggestion Reid had breached his home detention conditions. He had reported to his probation officers the day before, and was permitted to travel to the construction site for work.

Two people dead, six injured

Coster says the families of the two workers who were killed were working with police and expected to make formal identifications today.

"But we believe we know who they are," he says.

By early Friday, the conditions of those who had been injured ranged from serious to moderate, Coster says.

"[It's] a terrible time for them and their families."

And, of the two police officers injured, one remained in hospital in a stable condition.

"I expect he'll be there for quite some time, he has quite a road to recovery."

Police spoke to about 70 witnesses by the end of Thursday, and says it was likely the gunman knew the people who were killed.

"We are of the understanding ... that this is related to an issue at the workplace. The team will be piecing together the fine detail of what that was, but on the surface it related to some workplace tension."

The investigation would include a post-mortem, and a detailed scene examination. Coster says the fact the site was under construction would present safety issues that police would have to work carefully with.

Police in central Auckland the day after the construction site shootings. Photo: Charlotte Cook/RNZ.

Auckland central left reeling as football world cup begins

The opening match of the FIFA Football World Cup went ahead at Eden Park on Thursday evening, about 4km from the where the shooting took place, despite today's shooting in Auckland.

Minister of Police Ginny Andersen says she had spoken with police about safety measures.

"I want to reassure the central Auckland businesses that the central business district is safe.

"I was reassured that not only are police integrated into the whole organisation of the FIFA World Cup, but also there's extra presence in and around that [Central Business District (CBD)], not just at that site of the fatalities, but right around the CBD," Andersen says.

The impact of yesterday's events would be wide ranging, she says.

"We have lost two lives yesterday, those people went to work and did not come home to their family.

"It has been a shocking time for Auckland CBD. And the actions of the police - the fact that they were there within 11 minutes of that phone call being made, kept New Zealanders safe, and their quick action and their bravery did save lives."

Most of the streets in Auckland central that were closed by police on Thursday had been reopened by Friday, though there were still some disruptions directly around the construction site, Police Commissioner Coster says.

Officers were prioritising finishing the site examination everywhere except inside the building that is under construction, so the rest of the area could return to normal as much as possible in the next day or two.

Ngāti Whātua Ōrākei says on Friday that there was no rāhui in place in connection to the incident.

Home detention and reoffending

Asked about whether there were issues with the number of criminals convicted for violent offences being given sentence discounts for factors like pleading guilty, or being given home detention instead of serving their sentence behind bars, Minister of Police Ginny Andersen says it was important to look at the detail.

"The two key areas that we've seen a reduction in prison numbers is low level drug offences, such as cannabis, and also traffic offences - they're the two big decreases.

"So there has not been a decrease in violent crime receiving a prison sentence."

She says it was too early to comment on the fact Reid had been on home detention when the shootings took place. That would be considered as part of investigations into the incident, including by police and an investigation by the Department of Corrections "to detect if there were any flags that could have been missed".

"We really need to have all of the information in front of us in terms of what happened in this particular instance, to understand if the system can be strengthened...," Andersen says.

Police commissioner Coster says having more offenders on home detention inevitably meant police were more likely to deal with crime committed by them.

"Home detention was introduced about 10 years ago ... and the numbers there have grown over time. That isn't necessarily a problem, it's really about what happens with those who breach and how they're managed.

"There are a large number of people on electronically monitored bail, on home detention. Many of those sentence are served without significant issue.

"Certainly anyone who's on that sentence is going to be a higher risk of reoffending than the average person, so we do see that occurring. [But] the judgements about whether someone should be [on home detention] or in prison are not matters for police, they're for the judiciary, but that is something that we manage."

- RNZ.