When British backpacker Grace Millane went missing, investigators quickly turned to CCTV.

Within a day, they had released a grainy security camera image showing Millane at SkyCity as they appealed for public help in finding her.

Two days later, investigators had found her body and charged a 26-year-old man with her murder.

CCTV footage played a "crucial" role in the investigation by helping trace Millane's final movements, police said in a media release at the time.

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While only a small percentage of the more than 5000 CCTV cameras trained on Auckland's city streets, rail networks and council property are used for public safety, those few "prove their worth time and again", retired Detective Inspector Graham Bell said.

As Millane's grief-stricken family get set to bury the 22-year-old in the UK this week and New Zealand continues to mourn her death, the Weekend Herald looked at how CCTV may have been used to uncover her final steps.

Police say Millane was seen on CCTV footage leaving Andy's Burger Bar at SkyCity on December 1, four days before she was reported missing to police. She was then seen again that night at CityLife Hotel on Queen St.

A Weekend Herald survey of nearby streets shows as many as 16 public CCTV cameras could have picked up footage of Millane walking between the two places.

The most dense network of cameras are at SkyCity, where more than five keep an eye on the public coming and going along Victoria and Federal Sts, past the Sky Tower.

If Millane then walked from there down Victoria and Queen Sts to CityLife Hotel, she would likely have been picked up by cameras mounted on poles and traffic lights at every intersection along the way.

The Victoria and Queen St intersection - as one of Auckland's busiest - has at least four CCTV cameras alone.

Had Millane taken a route to CityLife Hotel through the back lane of Durham St West, she would likely have shown up on CCTV cameras at SkyCity and Albert St, before CityLife's own security cameras captured footage of her.

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Auckland Council service integration manager Duncan McLagge said Auckland Council has around 2000 cameras, while Auckland Transport has about 3200 cameras, including 950 on the rail network.

However, only a very small percentage were used for public safety.

Police would not comment on the investigation into Millane's death because it was before the courts and because of "operational reasons" they could not go into detail on how CCTV cameras are used to assist investigations.

But Auckland City Police Inspector George Fanamanu said police ask permission to access cameras owned by local councils, business associations and the NZ Transport Agency that have been installed in public places for crime prevention.

Millane was seen on CCTV footage leaving Andy's Burger Bar at SkyCity. Photo / Supplied
Millane was seen on CCTV footage leaving Andy's Burger Bar at SkyCity. Photo / Supplied

They have permission to monitor some of the cameras live.

"In general, police may access CCTV cameras that are operated by local authorities and other groups including NZTA and local business associations, who have installed CCTV in public places for crime prevention," Fanamanu said.

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He said police recognised "that the sharing of CCTV data is only permissible in accordance with the Privacy Act 1993".

Former investigator Bell said the first step in a missing person case was finding out where they were last seen.

"Once you find that, then you can work back in both directions from there," he said.

"So for Grace, how did she come to be there, where did she come from and where did she go afterwards."

Members of the public pay their respects during a vigil for British backpacker Grace Millane at Civic Square in Wellington. Photo / Getty Images
Members of the public pay their respects during a vigil for British backpacker Grace Millane at Civic Square in Wellington. Photo / Getty Images

That is where CCTV cameras are so important.

They help investigators find out who has been with the missing person, what the missing person was wearing and how they looked at the time, such as were they happy or under duress.

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As in Millane's case, stills from the footage can then be released to the public along with appeals for information or sightings.

Bell said his 13 years as a presenter on Police Ten 7 helped bring home how successful these images were in drawing information from the public.

CCTV images came with accurate time and location logs. When this was combined with other electronic data, such as mobile phone data and credit card and Uber transactions, an accurate picture could be built of where a person has been.

We look at how CCTV footage was used to trace Grace's final steps. Photo / Facebook
We look at how CCTV footage was used to trace Grace's final steps. Photo / Facebook

"Times and places become very important when you get to criminal trials because they improve the case," he said.

"Often you have to pin down the exact timings and movements of not only the offender but the victim as well."

And unlike the memory of a witness, defence lawyers find it harder to challenge times and dates recorded by CCTV cameras.

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Yet Bell also cautioned against expectations technology alone could solve crimes.

"One of the things police battle with these days is the unrealistic expectations created by television programmes, like CSI, where DNA tests come back within seconds and computers do impossible things," he said.

A spokesman for the Office of the Privacy Commissioner said one of the guiding principles for using a CCTV camera under the Privacy Act was that there be a "clearly defined" purpose. "That purpose might be to improve public safety, and monitor and collect evidence of crimes. Other purposes might be to monitor foot and vehicle traffic to understand choke or congestion points," he said.

Bell, meanwhile, said the quick arrest in the Millane case helped show how important CCTV cameras had become.

"It is an electronic world we live in and although some people might not like the fact our movements can be traced, actually it is a safeguard for us all."