A man who tried to blackmail a 14-year-old girl into sending him naked photos has been jailed for six years, with a minimum non-parole period of 30 months.

Karl Sheridan Salmon of North Canterbury previously admitted having sex with the underage girl and trying to use a video of the encounter to get her to send him naked photos, among charges including unlawful sexual connection, blackmail and threatening to kill.

At his sentencing in the High Court at Christchurch this morning, Justice Cameron Mander told the 21-year-old his actions were predatory.

He said Salmon felt more powerful on social media than in real life, and could say things through texts and Facebook that he couldn't in person.

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"The use of blackmail, and the manipulation of your victims by having them fear you, is conduct that is neither spontaneous nor entered into without some thought."

Two of Salmon's victims were in the public gallery and cried as the summary of facts was read.

Crown prosecutor Claire Boshier said they would suffer lasting trauma.

"There were serious effects, with some victims considering suicide and self-harm, and others will continue to be affected by this for quite some time."

A psychiatric report found there was a high risk of Salmon carrying out further sexual offending.

Ms Boshier told the court Salmon had shown next to no remorse.

"His insight into the impact of his offending on his victims appeared to be lacking, and he appeared to minimise this."

Salmon said he would have never followed through with his threats.

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* On September 11, 2015 the Herald stated that Karl Salmon had pleaded guilty to a charge of rape in the proceedings reported above. In fact he pleaded guilty to a charge of sexual connection with a girl under the age of 16 years after having intercourse with her.