A forensic scientist says DNA found on the memory card of a hidden toilet camera at New Zealand's embassy in Washington is "10,000 million times more likely" to belong to the country's top military attache.

Former Assistant Chief of Navy Alfred (Fred) Keating is accused of planting the covert device in a unisex bathroom in the embassy in the American capital during July 2017.

The former commodore was the senior defence attache to the US at the time and one of the New Zealand military's highest-ranking officers.

The 59-year-old is now on trial in the Auckland District Court this week, charged with attempting to make an intimate visual recording of another person.

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After being discovered by embassy staff, the camera and its memory card were sent to Wellington in a sealed diplomatic package for Constable Richard Matthews to examine for DNA samples.

Environmental Science and Research forensic scientist Sue Vintiner told the court this afternoon an "ultra-sensitive" DNA test was then conducted on the swab samples.

"When you touch something there is a good chance that you will leave your DNA behind," she explained.

A test of the memory card showed two DNA components, one being that of a man, Vintiner said.

The other was of an unidentifiable person, she said.

The memory card sample was then compared to the DNA of the investigation's suspects and Vintiner said the male profile corresponded with that of Keating's.

"This means Mr Keating could be the source of this DNA or someone else could be the source who has the same DNA profile as Mr Keating," she said.

However, Vintiner continued, the statistical likelihood of it being Keating's DNA was "10,000 million times more likely" than another person with the same DNA profile.

She said this was "extremely strong scientific support" that the DNA found on the memory card originated from the former commanding officer of the Devonport Naval Base.

But Vintiner conceded a person's DNA can also be transferred to an object by another person who had touched that object.

Alfred Keating was a commodore and one of the navy's highest-ranking officers. Photo / Supplied
Alfred Keating was a commodore and one of the navy's highest-ranking officers. Photo / Supplied

Earlier today, a secret witness gave evidence in a locked courtroom, with members of the public and press ordered to leave the trial for national security reasons.

The witness' identity has been suppressed by Judge Robert Ronayne, who earlier warned the jury about some of the evidence they would hear during the two-week trial.

The judge said it was "terribly important" that no classified material or court documents were taken home by the jurors and nothing discussed outside of the jury room.

Richard Kay, the unit manager for the Ministry of Foreign Affairs and Trade (MFAT) Americas division, told the court today the camera's discovery at one of New Zealand's largest and most critical diplomatic buildings was "shocking".

After initially investigating the find with a task force the decision was made to hand over responsibility to the New Zealand Police, Kay said.

"It was such a serious matter and such a shocking discovery ... the police had the best skills to handle such a matter," he said.

Fellow MFAT employee Angela Woodham, the operations manger responsible for the embassy's security, also recalled when Stephen Warren alerted her to the potential security breach.

Warren, a former police officer and Defence Force staffer at the embassy, told the court yesterday of the "black box" he saw in the bathroom.

After seeking advice from Daniel Vrink, a colleague with a higher security clearance, Warren reported the find to Woodham.

"I had a gut feeling that the black box was something more than a hard drive," Warren said.

Woodham said she then searched the internet for the brand name, BrickHouse Security, which was inscribed on the device and secured in a locked cabinet.

"I took hold of it, had a look at it, Googled the brand that was on the side ... As soon as we learnt that it was a hidden camera I put a post-it note over the hidden lens."

Alfred Keating is accused of attempting to make an intimate visual recording of another person at the New Zealand embassy in Washington DC. Photo / Supplied
Alfred Keating is accused of attempting to make an intimate visual recording of another person at the New Zealand embassy in Washington DC. Photo / Supplied

Woodham later contacted Detective Inspector Neil Hallett, the senior police liaison officer in Washington who oversaw New Zealand's police matters in the US, Canada and Central and South America.

The detective quickly arranged a meeting with Woodham and then Ambassador Tim Groser at the embassy, where Hallett took possession of the device before beginning an investigation, the court heard.

Woodham also explained that Keating was on an A-1 visa and diplomatic passport while stationed in the US, giving him full privileges and immunity from American prosecution.

She added Keating made a passing comment to her about a camera as he walked past her office at the embassy during the early stages of the investigation.

Keating's lawyer Ron Mansfield, however, argues the Crown's case is circumstantial and weak.

"The evidence doesn't tell you who did it and it certainly doesn't tell you it was Mr Keating," Mansfield told the jury.

Keating pleaded not guilty last March and two days later resigned from his post in the NZDF, ending a more than 40-year career.

The military leader from Northland also served as New Zealand's naval attache and senior technical officer for the navy to the US from July 2003 until December 2006.