A man who impersonated a teenage boy on Facebook to befriend and then blackmail three girls has escaped a prison sentence.

Brandyn Alan Stewart was sentenced by Justice David Collins to 11 months home detention in the High Court at Wellington today.

He had earlier pleaded guilty to three blackmail charges and two indecent act charges.

In his sentencing notes, Justice Collins said Stewart had assumed fake identities in which he said he was 16-years-old, when in fact he was about four years older.

He befriended girls aged 11, 14 and 16 - although at the time Stewart thought the 11-year-old was 14.

"After you befriended each of the complainants you persuaded them to send you compromising photographs of themselves in which they were either naked or dressed only in underwear,'' Justice Collins said.

"You continued to groom the complainants and threatened each of them with disclosing their photographs online unless they complied with your demands.''

Stewart asked the 16-year-old to meet him for sex, demanded the 14-year-old send more naked photographs of herself, and instructed the 11-year-old to continue correspondence with him.

The messages to the girls from Stewart were "persistent'' and conveyed over Facebook and through text messages.

On one occasion Stewart did send a photo of the 11-year-old to a friend of hers, which in turn made her think he would follow through with the threats, Justice Collins said.

"Thankfully, in no case were your demands ultimately successful.''

The judge noted Stewart had been diagnosed with Attention Deficit Disorder.

A clinical psychologist had earlier told Justice Collins the disorder "significantly'' affected Stewart's behaviour and contributed to impulsiveness and a disregard to the consequence of his conduct.

While serving his sentence, Stewart was also ordered not to drink alcohol or take drugs, to attend psychological counselling and take drug and alcohol counselling.

He was also banned from accessing the internet unless an approved supervisor was with him and banned from contacting anyone under 16 years of age except in the presence of an adult who knew his background.

Justice Collins sentenced Stewart to 11 months home detention on the charge relating to the 16-year-old and six months home detention for the other charges, to be served concurrently.