A teenage student planned to shoot teachers and fellow pupils at a school until police were tipped off to disturbing social media posts.

The student, who was 17 at the time, had a pistol-grip shotgun and military-style semi-automatic rifle in their bedroom, as well as three "improvised explosive devices" made from gas canisters filled with gunpowder.

When police raided the teen's home they found a hand-drawn diagram of a school building with "prime targets" marked with an X, including staff offices, and arrows to show planned movements along corridors.

The disturbing details can be reported for the first time today after the NZ Herald and Stuff fought a 22-month legal battle to overturn blanket suppression orders, which prevented any reporting of the case since the arrest.

Detectives were tipped off to the teen's "confrontational" posts on social media and searched their home, where they found documents the student had written including one titled "Note to investigator".

It was a letter in which the student described themself as a "terrorist", explained their actions and outlined the steps taken to prepare for the shooting, such as the collection of ammunition.

"Of concern to the police is that the equipment necessary to carry out [the] plans had been obtained by the defendant and located at the address," the police wrote in a summary of facts, noting the firearms, ammunition, explosives, clothing, knives and sketches of the school.

The purported "terrorist attack" plan was discovered only after members of the public tipped off the police about the social media posts.

These comments referred to blowing up a school and executing a shooting, as well as posting increasingly "inflammatory and extreme views".

The student drew sketches of the school buildings with proposed movements and targets. Illustration / Andrew Louis

The student also posted online pictures wearing a military-style tactical vest, military-style gloves, and a holster capable of holding the shotgun.

In some photos, the teen was holding the shotgun which had been lawfully modified by replacing the butt with a pistol grip.

Following the arrest last year, the student's peers told police the 17-year-old made comments about "blowing up" the school stage and shooting people in assembly.

The police also seized electronic devices in the student's home such as a laptop, iPad and gaming devices.

They found hundreds of images of female children wearing bikinis, or in sexual poses, or being sexually abused.

The pupil was charged with unlawful possession of the Mossberg shotgun and unlawful possession of an AR-15 semi-automatic rifle.

The home-made bombs were powerful enough to blow off someone's hand, according to a Defence Force bomb disposal expert.

The student admitted making them, and setting one off in an abandoned building.

A charge of unlawful possession of explosives was also laid, as well as 19 counts of possession of objectionable material, in relation to images of young girls found on the computer.

The self-described 'terrorist' was armed with a pistol-grip shotgun and homemade bombs. Illustration / Andrew Louis

Now 18 years old, the young person pleaded guilty to the charges and was sentenced last December to six months' community detention and two years of intensive supervision, with 15 strict conditions.

The former student's movements are monitored by GPS, they cannot use internet-capable devices, and must attend counselling and therapy sessions with clinical professionals.

"I have done, and others who are trying to help you have done, what they can. It is now over to you. I wish you good luck," Judge David Ruth said in his closing remarks at the sentencing hearing in December.

"I do not want to see you back here again. I want you to have a good life, free from all of this, but it is now up to you. We have done all we can. Work with the people who want to walk with you, and get on with a better life."

In later seeking an order for permanent name suppression, the pupil's defence lawyer, Robert Lithgow, QC, said the progress his client had made during rehabilitation would unravel if their identity was made public.

If that happened, Lithgow said the case would attract "unwanted notoriety" and "misguided excitement" from radical extremists which would be a "nightmare" for the family, and New Zealand society if it encouraged copy-cat behaviour.

The teen's family also believed they would have to leave their hometown if their name was made public, said Lithgow, as they feared being blamed for the student's behaviour and driven out of the community.

Lithgow said his client would become reclusive if named and stop engaging with the clinical professionals, or seeking future employment and study opportunities, which would increase the risk of reoffending.

His arguments convinced Judge Ruth to permanently suppress the name of the former student, as well as the school, at a hearing in July this year. It can be reported the school is in the Tasman area.

"I think it would be unconscionable for this court to order that there be publication, or not grant permanent suppression to put it more accurately, in the knowledge that every agency that has dealt with this young [person] is of the view that it would certainly destabilise the rehabilitative efforts that have been put in, and will continue to be put in, for this young [person]," said Judge Ruth.

"My view, as to risk, is that I think it would be a high risk of offending of the type which the defendant may have been contemplating in [their] writings if there is publication. It is simply not a risk that this court should take."

The teenager wrote a document to police which explained the planned 'terrorist attack' and steps taken, such as collection of ammunition. Illustration / Andrew Louis

However, Judge Ruth did agree to lift the blanket suppression orders to allow some - but not all - details of what he described as a "most unusual" case to be reported.

NZME, the publisher of the New Zealand Herald, and the Stuff website appealed this ruling in a hearing at the High Court last month to argue that the full circumstances of the case should be reported.

However, Justice Francis Cooke dismissed the appeal except to make some minor changes to the agreed summary of facts in a judgment released yesterday.

All other evidence and facts in the case are suppressed, ruled Justice Cooke in a decision which lifted the shroud of total secrecy imposed on the case for the past 22 months.

There is no way of knowing whether the student would have gone ahead with the plot, or if they would have been successful in executing staff and students in a mass school shooting.

Such acts of extreme violence are more often associated with the United States, such as the Sandy Hook and Columbine massacres.

But at an earlier court hearing, Judge Ruth dismissed any attempt to downplay the student's behaviour as "mere musings of a disaffected teenage malcontent", as the student's preparation went "way beyond any suggestion of that nature".

The judge also wanted to "make clear" any heightened sensitivity following the Christchurch mosque shootings played no part in his thinking.

Shortly after the student's arrest, 51 people were murdered while worshipping at two mosques in Christchurch on March 15, 2019. The mosque shooter, Brenton Tarrant, has now been sentenced to life imprisonment without parole.

At the teenager's sentencing indication hearing in September last year, Judge Ruth said the terror attacks in Christchurch were "irrelevant" to his decision, and if anything, the actions of police may have prevented a similar tragedy.

"I reached a view much, much earlier than the March incident that this was a matter of considerable seriousness," said Judge Ruth at a hearing in September 2019.

"I share the view expressed by the police that, on the face of it at least, this is a very disturbed and troubled young [person].

"My only reference to the March 15 episode is that perhaps by incarcerating this young [person] and taking the cautious and conservative approach the court has, it's arguably the case that we prevented yet another episode similar to those that did occur in Christchurch."

The teenage student wore tactical military clothing to pose with a shotgun in 'confrontational' social media posts. Illustration / Andrew Louis

And similarly to the Christchurch terror attack, this case raises serious questions around the vetting of firearms licence holders.

The student held an A category firearms licence and owned the Mossberg shotgun, a birthday present, which was stored in a locked cabinet in their bedroom.

Police seized the shotgun along with a Colt AR-15 semi-automatic rifle also stored in the locker.

Under the law at the time, anyone with an A category licence could possess an AR-15.

However, this particular model – which had been loaned to the family to check before they purchased it – had been modified with a flash suppressor and bayonet lugs.

This meant the AR-15 was technically a military style semi-automatic (MSSA) which needed an E category endorsement on the licence, which the student did not have.

Lithgow said neither his client, nor the AR-15's owner, was aware the firearm needed an E endorsement.

The AR-15 has gained notoriety as the weapon of choice for mass shootings around the world, including the March 15 terror attacks in Christchurch.

Following the mosque shootings, the Government banned semi-automatic firearms such as the AR-15.

Assistant Commissioner Mike Johnson said complaints from the public in 2018 and 2019 were taken "extremely seriously", which allowed the police to arrest the teen and "defuse any threat to the community".

"The case highlights the significant value of information provided by the public and we thank all those who reported their concerns to police," said Johnson.

"It also demonstrates how important it is for parents and caregivers to keep a close eye on what young people are doing online."