A crown case against a Tasmanian man accused of trying to procure a teenage boy for sex collapsed after a judge ruled evidence from a self-proclaimed "paedophile-hunter" was inadmissible.

Key points: Judge ruled filming of man lured into thinking he was hooking up with a teenager was not admissible evidence

Judge ruled filming of man lured into thinking he was hooking up with a teenager was not admissible evidence He said the vigilante instigated the commission of the same crime he was trying to expose

He said the vigilante instigated the commission of the same crime he was trying to expose He warned such vigilante actions could expose people to violence and impede legitimate investigations

The man from Beaconsfield had pleaded not guilty to a charge of communicating with intent to procure a person under 17 years to engage in an unlawful sexual act.

The charge was dropped on May 17 after Chief Justice Alan Blow refused to admit evidence from the Crown's principal witness, who described himself to police as a "paedo-hunter".

According to Chief Justice Blow's May 7 judgment in the Supreme Court in Launceston, the witness was masquerading online as a 14-year-old boy called "Josh" in late 2016 when he made contact with the accused man through the dating app Grindr.

The two men continued communicating on another app, Kik, exchanging messages in which first the accused and then both men discussed engaging in sexual activity.

The witness arranged to meet the accused man at Greens Beach and then filmed the confrontation and put the video on YouTube.

After police contacted him, the witness gave them screen shots of his conversations with the accused man and gave evidence at a preliminary proceedings hearing.

Chief Justice Alan Blow says some of the messages sent by the witness constituted child exploitation. ( ABC )

Chief Justice Blow found although the witness had "noble intentions" of child protection and public benefit, his actions had instigated and incited the commission of the same crime he was trying to expose.

Chief Justice Blow described the witness' contraventions of the law as "grave".

"This was a case of entrapment of a person apparently willing to engage in sexual acts with a 14-year-old boy, but in circumstances where there were risks of violence, property damage, harassment or suicide," he said.

Chief Justice Blow also found some of the messages the witness sent constituted child exploitation material because they described sexual activity involving a person under the age of 18.

The judge acknowledged excluding the witness' evidence would probably make the Crown's case against the accused man unviable.

"It is true that this was important evidence of the commission of a crime, without which the Crown will not be able to proceed," he said.

Judge criticises 'maverick vigilante' actions

Chief Justice Blow said his decision was also based on the dangers posed to the community by vigilante actions like those undertaken by the witness.

"It is significant that, if I were to admit the evidence, that might be regarded by some as this court giving some sort of approval to the sorts of methods used by the witness in this case," he said.

Chief Justice Blow said the witness' actions were likely to arouse public hostility towards the accused man and his family.

He said the dangers of denouncing people through social media included exposing the denounced person and their family to violence and property damage, exposing the vigilante to violence, driving someone to suicide and undermining or impeding police investigations.

"A big investigation could easily be compromised if a vigilante were to draw attention to an individual under investigation," he said.

He cited another case in which a police officer in the ACT posed as a child online in order to catch and prosecute paedophiles.

"That case illustrates the difference between a professional police investigation governed by appropriate guidelines and an investigation by a maverick vigilante intending publicity likely to cause hatred."