A teenage boy who says two police officers kidnapped him, one allegedly threatening him with a fabricated rape charge, is having his underage relationship scrutinised by a top lawyer.

The young Auckland man, now 19, alleges an inspector and a sergeant locked him in a prison cell, threatened him with a rape charge, and "deported" him to Australia to end his relationship with a 15-year-old girl in 2015.

Inspector Hurimoana Dennis, 52, and Sergeant Vaughan Perry, 45, are on trial in the High Court at Auckland before a jury and Justice Edwin Wylie charged with twice kidnapping the then 17-year-old.

Yesterday, the teenage complainant, whose identity is suppressed, described the alleged offending from the witness stand.

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Today, he is being cross-examined by Dennis' lawyer Stephen Bonnar QC.

Bonnar asked the teen to recall when he first started having a sexual relationship with his girlfriend, and if he understood it was potentially a crime.

The teenage boy's mother had filed a formal complaint with police about her son, alleging he was having a sexual relationship with a minor. However, police decided against laying charges.

The teen declined to answer Bonnar's repeated questions about underage sex, and Justice Wylie reminding him that he and was not required to give a reply that could incriminate him without first seeking legal advice.

The teen has previously denied having a sexual relationship with his girlfriend.

"This is a court of law. Do you know how precise we have to be?" Bonnar continued to push the teen.

The Queen's Counsel also questioned whether the teen understood his family and Dennis were concerned about his relationship.

"Yes and no," the young man replied.

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"Do you accept they were very concerned about underage sex? They were concerned about your welfare weren't they? And they were concerned about [your girlfriend's] welfare - she was the underage girl," Bonnar said, which the teen accepted.

The court has heard that the two teens are still in a relationship.

On May 5, 2015, after tense moments between the two teens' families, Dennis, a family friend and the police national Maori strategic adviser, organised a "mock arrest" to scare him off the underage affair.

The boy was processed at the Auckland Central Police Station as though he was being held in custody by Dennis and Perry, the duty custody sergeant.

The court heard that while in the holding cells, Dennis gave the teen an ultimatum.

"He said to me I could either go to Australia; or option two, he would take me downstairs and I'd be charged for statutory rape," the teen said.

Bonnar continued with his cross-examination of the teen and asked: "Did you think at any point that Hurimoana Dennis was ever trying to just send you a message or just trying to scare you?"

The 19-year-old replied that it didn't feel like a message, but "it did scare me big time".

However, in a police interview the teen had said it felt as though the inspector was trying to send a message and scare him about the potential consequences of an underage relationship.

"I just want to make it clear what the defence position is on this is. Virtually none of what you say that happened down in the cell blocks is true. I take it that you disagree with that?" Bonnar quipped.

After agreeing to leave for Australia, the young man was escorted to Auckland Airport and placed on a flight to Sydney.

Then on June 10, 2015, the teen tried to "escape" and return to New Zealand.

But Dennis "swung into action", the court has heard, contacting police at Auckland International Airport and arranging for armed police to usher the teen off the aircraft.

Dennis told the boy not to come back to the country until he was 18 and placed him on another flight to Australia, the court has heard.

After returning to Sydney, the teen claims he was assaulted and fled to friends of his girlfriend's mother before reporting what had happened to New South Wales Police.

Dennis, who was also the chairman of Te Puea Marae in Mangere, was stood down from his duties in September 2015 shortly after the teen laid a complaint.

The trial continues.