A man described as "just a country kid" did so well at growing cannabis in a secret room hidden behind a false wall that his lawyer suggested he might switch careers from forestry to horticulture.

Pax Brian Rasmussen's sophisticated growing operation was found by police after they arrived at his house to issue a temporary suspension of his firearms licence.

It was not disclosed to the court what had prompted this, but the police turning up led to charges including possession and cultivation of cannabis and failing to comply with a direction to surrender a firearm.

Rasmussen, 21, admitted the charges in the Nelson District Court today.

On October 20 last year, a senior sergeant from the police arms safety and control division issued a notice of temporary suspension while Rasmussen's firearms licence was considered for revocation.

Two days later the police arrived at his home to issue the notice and seize Rasmussen's firearms and licence.

The police summary of facts said he willingly surrendered his licence and told police he had no firearms in his possession.

Judge Jo Rielly said in sentencing that a police search of the address was triggered when they noticed a cabinet when they were standing talking to Rasmussen, which led them to believe firearms were on the property.

During the search, police found a Remington shotgun under his bed.

Ammunition was also found, including shotgun shells and .22 calibre rifle ammunition.

As the search continued police found six small cannabis seedlings in Rasmussen's bedroom in a small hydroponic tent.

An additional grow operation described as "quite sophisticated" was found behind a false wall which led to a specifically designed room under the house.

Six large cannabis plants were found, along with hydroponic growing equipment including lights, complex drainage systems and power sources through to a separate room.

His lawyer Tim Spear said Rasmussen worked in forestry and has been a keen hunter most of his teenage life.

He said the shotgun found under Rasmussen's bed was locked and hidden.

"This was not a situation where he had open firearms."

Spear said Rasmussen had "decided to try his hand growing cannabis".

"Basically, he's just a country kid working in forestry and who likes hunting and who then decided to grow a bit of cannabis.

"He did it quite well – perhaps he should go into horticulture," Spear said.

Rasmussen told police he failed to surrender the arms as he thought he could get away with it and didn't want to get into trouble for incorrect storage.

Judge Rielly said the fact that the firearm found under the bed was locked did mitigate the risk, but Rasmussen's failure to disclose he had it was "dishonest, reckless, entirely inappropriate and unlawful".

"There's a real concern in our community about people being in possession of firearms."

She said the argument presented that he has "decided to give cannabis a whirl" because he lived in the country inferred it was likely that Rasmussen was growing it for his own use.

However, the circumstances under which the cannabis was being grown were concerning, Judge Rielly said.

She said it was a deliberate attempt to grow what was still an illegal drug in New Zealand, and while there were not a lot of plants found, the larger ones hidden behind the false wall had reached maturity and indicated they might have yielded a high amount of cannabis head.

Judge Rielly said in sentencing Rasmussen that she hoped he would reflect on what he had done and carry on in a field in which he was described as a "very hard worker".

On the charge of cannabis cultivation, he was convicted and sentenced to 100 hours' community work. On the charge of failing to comply with the notice he was convicted and fined $800 and on the charge of cannabis possession he was convicted and discharged.

An order was made for the destruction of the firearm plus the drugs and equipment.