SYDNEY—With little more than an internet connection, a laptop and a cellphone, Chan Han Choi, a soft-spoken janitor in his early 60s, allegedly played a central role in a plot to help North Korea sell missile parts and coal on the black market.

Working from a rented apartment in a Sydney suburb, the South Korean-born Australian citizen spoke in code while negotiating with groups suspected of working on Pyongyang’s behalf, police said, adding that he used botanical terms such as “little pine tree” to describe missiles and “nursery” to refer to weapons factories.

These and other details—contained in investigation documents prepared by Australian police—are being used by prosecutors to paint a picture of an eager foot soldier for North Korean dictator Kim Jong Un who operated undetected and generated important income for Pyongyang for years before international sanctions were tightened.

The police and prosecutors allege he has been an economic agent of North Korea since at least July 2013 and brokered commodity sales for Pyongyang as early as 2008, before tighter international sanctions were in place.

Handcuffed and accompanied by three prison guards, Mr. Choi appeared in a Sydney court on Dec. 5. Dressed in a green prison sweatshirt, slacks and green sneakers, he spoke softly to a translator, self-consciously combing his hair with his hand.