UPDATED 12.06PM: A North Canterbury man who tried to blackmail a 14-year-old girl into sending him naked photos has been sentenced to six years in prison, with a non parole period of two and a half years.

Karl Sheridan Salmon admits having sex with the underage girl, and trying to use a video of the encounter to get her to send him naked photos.

He'd admitted a range of charges including unlawful sexual connection, blackmail and threatening to kill.

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

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

Justice Mander told the court Salmon felt more powerful on social media than he did in real life.

"You describe the lonely lifestyle to the pre-sentence report writer and claimed the threats communicated to your victims were empty, that you were capable of saying things in messages that you would not be able to say to others in person."

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

Crown prosecutor Claire Boshier says they will suffer lasting trauma.

"There were serious effects with some victims considering suicidal thoughts and self-harm. Others will continue to be effected."

A psychiatric report says Salmon is at a high risk of further sexual offending.