Father and son Michael Green and Joel St Claire partnered up in a 500-plant cannabis growing operation. A judge has ruled that they were motivated by personal medical use rather than financial benefit

Joel St Clair was hoping to reconnect with his ailing father when he moved into his home on Nelson's Tamaki Street.

But it wasn't long before the 34-year-old reluctantly agreed to help dad Michael Green with a project - cultivating 562 cannabis plants.

Green, 68, was growing cannabis to self-medicate and relieve his chronic back pain and cholangitis, an inflammatory disease of the liver.

The operation didn't stop there.

On January 13 last year police found what they described as a "substantial and sophisticated three-stage growing operation" at the Green's Tamaki St home and at a rental property on Valley Heights Road.

Three LSD tabs, 29 grams of cannabis resin, a vaporiser and spotting knives were also found at the Valley Heights Rd address.

Both men said the grow houses were theirs alone but in January a jury found the pair guilty on seven charges including cultivating cannabis and possessing it for sale.

The duo's denial that there was any supplying involved was rejected by the jury and Green bore full responsibility for the larger Valley Heights operation.

On Thursday they appeared in Nelson District Court where judge Arthur Tompkins said they posed "a low risk of reoffending and causing harm" and sentenced them to home detention.

Despite the jury's verdict and some small-scale sales, Tompkins said there was "no evidence of a financial motive for either defendant".

He said Green bore greater culpability for having enticed his son into the operation.

"Mr St Clair was reluctant to be involved, but once involved he was an enthusiastic and profligate user of cannabis."

Crown prosecutor Jackson Webber said both individuals had used large amounts of cannabis, primarily for medicinal use.

"The evidence that was given at trial by the doctor was that [Green's] cholangitis was intermittent and would flare up, it was not something that was constant."

Defence lawyer Tony Bamford said Green's primary motivation for the Valley Heights grow house was to have a regular supply of cannabis for his own medicinal needs.

"Ultimately it is an issue of extensive, heavy, personal use with an element of commercial supply," Bamford said.

"There are no obvious signs of significant commercial benefit."