An undercover police officer bought marijuana seedlings from a Switched on Gardener employee who closed the deal inside the company's Tauranga shop, a court has been told.

The undercover officer, who uses the name "Jack", wore a wire during the transaction and was giving evidence at the Auckland District Court today where Michael Maurice Quinlan - the owner of Switched on Gardener - and four of his colleagues are on trial.

Quinlan is alleged to have been the head of the operation that the Crown describes as a "one-stop shop" for cannabis growing equipment.

His two companies, Stoneware 91 Ltd and Hydroponics Wholesalers, together with general manager Peter John Bennett, business development manager Ricky Cochrane, distribution manager Andrew Barry Mai and the South Island manager Paul Kenneth Barlow are accused of belonging to an organised criminal group and supplying equipment used to grow cannabis.

All have pleaded not guilty to the charges.

"Jack" told the closed court today that he bought 10 seedlings off one of the staff members at the Tauranga shop.

The woman, who introduced herself as Marie quoted a price of $20 a plant. She threw in an extra one because one of the plants had yellowing leaves. She told Jack "that's how I roll".

"Jack" also visited a shop in Upper Hutt where he told a Switched on Gardener employee that his mate had left a couple of plants at his house but the leaves were starting to yellow and were curling up.

The man, who said his name was Kyle, gave the undercover officer advice on everything from ventilation and light to watering frequency and what to feed his plants. "I've been doing this for 20 years."

Kyle sold the officer a book on hydroponics and a packet of plant food.

There was also a discussion about how to conceal the smell of the plants with fans and carbon filters.

"Your big cost is your smell control but in saying that, it is the biggest thing that gives people away ... You can't miss the smell. You might have one plant that when people get to your letter box [they say]: 'Hey, I f***ing know what this guy's up to'."

"Jack" told the court that he saw marijuana bongs inside the shop, a magazine called "High Times" was for sale, and plant food called budzilla and head-master was also available.

Under cross-examination from Quinlan's lawyer, Paul Davison QC, the officer was asked about a conversation recorded at the Wellington Switched on Gardener shop that featured the officer asking for advice about a "couple of dak plants".

The shop worker replied: "We can't discuss anything like that ... I can't really talk to you ... You've just told me what you're doing."

Mr Davison asked the officer why he had not included the conversation in his report and said the omission was "very selective and one-sided".

The officer replied that the recording was included in his evidence and he felt it spoke for itself.

The trial, before a jury of eight women and four men, is set down for eight weeks.