The investigation began in March this year, when a 24-year-old man was arrested in Langwarrin.
Police said he had an imitation handgun on him.
Police then searched a factory in Carrum Downs on May 11, seizing two handguns, including one that had been 3D-printed.
A 32-year-old man was arrested then and charged with firearm and drug offences.
He was later sentenced to four months behind bars.
The final phase of the investigation was carried out this week, when police searched a Cranbourne East house on Thursday, August 17.
Police seized a number of items, including five 3D-printed handguns, including one designed to fire shotgun rounds, one semi-automatic handgun, additional 3D-printed gun parts, ammo, and a 3D printer.
A 31-year-old male was arrested at the house and has been charged with manufacturing and trafficking firearms.
He was remanded to appear in court on September 12.
In total, the investigation saw nine guns seized, including six 3D-printed ones.
Quotes attributed to Senior Sergeant Stephen Fyffe
“We have dismantled a local firearms manufacturing enterprise this week and taken nine illicit firearms off the streets,” Senior Sergeant Stephen Fyffe said.