Police seized about $5.66 million of equipment and material, including 1603 cannabis plants and $8900 cash, when they searched a George Street property last February.

The cannabis was growing in an elaborate hydroponics operation in a tunnel once used by Roads and Traffic Authority staff to cross the highway safely.

Raymond Henderson, 54, from Conder in the ACT, has pleaded guilty to multiple drug supply, cultivation and possession charges.

Police began surveilling the two-storey house in Marulan that Henderson was renting and growing cannabis in on February 12 last year, per police facts tendered in Goulburn Local Court.

On February 13, 2019, about 8.25am, police caught Henderson speeding in a red Dodge wagon on the Hume Highway at Mittagong.

When stopped, police searched the car and found cannabis inside a Jacob's Creek wine box and cash inside a red McCain spaghetti bolognese box.

They also found equipment used to grow cannabis, including a number of large tubs and vacuum bags.

Henderson was arrested and taken to Southern Highlands Police Station that morning and police issued a search warrant of his house for that evening.

In the underground tunnel, accessed by two sets of spiral stairs inside the house, police uncovered an elaborate hydroponics operation.

Cannabis plants were also located in a large shed separate to the residential building.

A complex system of pipes was used to water the cannabis plants, assisted by overhead heat lamps and fans.

There were numerous spare globes for the lighting system, bags of fertiliser, manuals on how to grow cannabis plants, timers and planners on the door.

A rifle was also found by police on a set of steel stairs that led to a mezzanine level of the building.

Henderson's case was mentioned in Goulburn Local Court on January 15 where no application was made for bail and bail was formally refused.

A date of February 3 has been set for when Henderson will be sentenced in Goulburn District Court.