Police found an arsenal of weapons at a Bay of Plenty teenager's house after hearing that a bomb might be detonated at a school prizegiving, Rotorua District Court has been told.

Among the weapons was a pipe bomb primed with gunpowder.

Daryl John Thomas, 17, of Reporoa, farm-hand appeared in court yesterday, pleading guilty to charges of possessing an explosive and possessing a firearm without a licence.

He was remanded at large until November 22 for sentencing.

Thomas was living with his parents on a farm at Reporoa. Police said they searched Thomas' house on September 6 after allegations a bomb might be detonated at the Reporoa College prizegiving.

They found an unlicensed loaded .22 rifle in Thomas' room and a pipe bomb filled with wadding and gunpowder in a shed .

Airforce personnel detonated the device and told police that anybody standing within a three-metre radius of the bomb would have been killed.

Police also found a number of whole and partly constructed weapons, including grenade launchers, crossbows, gunpowder and other items used in the manufacture of bombs.

Last year Thomas built his own website, detailing how to manufacture weapons and devices, police said. Thomas said building the devices was a hobby, which his parents knew about.

Thomas said he'd been a victim of rumours over the years and had never meant to hurt anyone. He co-operated fully with the police.

Thomas' lawyer Andy Schulze said there was a background to the offences.

Judge James Weir requested a full report to assist him in sentencing, saying it was not a straight forward case.

He said Thomas' unhealthy fascination with explosives and weapons of warfare was an issue.

He remanded Thomas at large for sentencing on November 22.