A former policeman's free sex with a prostitute he threatened has cost his reputation, his marriage, his career, and now it has drawn a two-year jail term.

Nathan Thorose Connolly has been publicly vilified and humiliated, and as an ex-cop in jail he can expect to spend most of his jail term in solitary confinement, in lock-down for 23 hours a day, the Christchurch court news website reported.

The High Court in Christchurch was told today that the sexual offence had ruined him financially and damaged his relationship with his two young children.

Justice Christine French said it had also damaged the police.

She told him: "The fact that you committed this offence as a police officer is an aggravating feature because of the harm done to the police and the resulting loss of public confidence.

"That is something that strikes at the very heart of our legal system. It is imperative that we have faith in the integrity of our police officers who are entrusted by the community with the task of enforcing the law."

Connolly, 31 and now unemployed, was convicted at a jury trial on one charge of inducing sexual connection from the sex worker by means of a threat. He was acquitted on two other charges.

The trial was told the pair had regular sexual liaisons, but things changed after she happened to visit the police station and saw that Connolly was a policeman.

He stopped her car and told her he could issue tickets totalling $1000 for not having a warrant or registration, and could have the car impounded. The tickets weren't issued, but the two later had sex. No payment of the $60 fee was mentioned.

Crown prosecutor Mark Zarifeh said: "The complainant's evidence was that she had sex even when he did not pay her because she was scared of the consequences if she did not do so.

Justice French referred to the vulnerability of the victim. "She had no-one she could turn to."

Defence counsel Jonathan Eaton said Connolly had been convicted of a single act at a time when he had made no overt threat.

"It was not a case where he became demanding or insistent," said Mr Eaton. "The sex they engaged in was 'normal' sex - there was nothing aggressive, demanding, or demeaning about it."

The only difference between then and their other acts of intercourse was that Connolly was on duty, in a police car, and the issue of money was not discussed at all.

He said Connolly had a lot of support. He had lost his marriage but his wife remained supportive in what had become impossible circumstances for her.

Connolly accepted and expressed sympathy and understanding of the position the victim felt she was in.

Justice French imposed a two-year jail term after ruling out home detention.

She said: "Home detention would be an inappropriate response to the seriousness of this offending involving a police officer abusing his power for his own personal benefit."

Outside the court, Anna Reed of the Prostitutes' Collective, said the victim was very relieved that the case was over because she had been under a lot of stress.

"What he did was very wrong," said Miss Reed. "Just because he is a policeman doesn't make it right."