A NSW South Coast gun shop owner who masterminded an illegal interstate firearms racket has pleaded guilty to supplying hundreds of guns over a six-year period.

Key points: Shane Simpson illegally supplied 279 firearms, including Glocks and Desert Eagles, to multiple people between 2013 and 2019.

Shane Simpson illegally supplied 279 firearms, including Glocks and Desert Eagles, to multiple people between 2013 and 2019. A Glock 17A pistol supplied by Simpson was used in the alleged murder of Craig Anderson, 51, in Sydney last May

A Glock 17A pistol supplied by Simpson was used in the alleged murder of Craig Anderson, 51, in Sydney last May Simpson claims he committed the crime to protect his family

Wollongong gun shop owner Shane Simpson illegally supplied 279 firearms, including Glocks and Desert Eagles, to multiple people including underworld figures between 2013 and 2019.

A Glock 17A pistol supplied by Simpson was used in the alleged murder of 51-year-old Craig Anderson in Doonside in Sydney's west in May last year.

The pistol, with a defaced serial number, was found in a vehicle after the alleged murder.

Six people, including a lawyer and an alleged Comanchero bikie, have been charged over the killing.

"[I] wasn't doing it because I wanted to do it … [I] just wanted to make sure my family is going to be OK," Simpson said.

Shane Simpson was arrested and charged by NSW Police following a raid on his gun shop at Port Kembla. ( Supplied: NSW Police )

Court documents today revealed the elaborate way Simpson distributed the firearms on the black market.

Using the front of his Port Kembla gun shop, Simpson Sports Pty Ltd, Simpson acquired hundreds of weapons and then on-sold them to unknown parties, police said.

He would then fraudulently update the NSW Firearms Registry to show he sold the firearms to interstate arms dealers, but they had no knowledge of this and never received the weapons.

In an interview with police, Simpson said he would trawl the internet looking for a random firearms dealer to attribute the sales to, or invent a fictitious one.

Simpson admitted he would use an angle grinder to remove the serial number from the guns, making them difficult for authorities to trace.

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Strike Force Myosoti was established to investigate the movement of firearms between states, and used forensic analysis to link Simpson with weapons used in crimes across the country.

In an interview with police after his arrest, Simpson said he had been "pushed and pushed for a long time".

Ten of the guns he sold were later linked to serious criminal activity, including the alleged murder of Mr Anderson and a drive-by shooting in Greystanes.

Simpson was initially facing hundreds of charges but the majority were withdrawn in court.

In Wollongong Local Court he pleaded guilty to just seven, including six counts of ongoing supply of firearms over six years.

Simpson is facing a maximum sentence of 20 years in jail.

He will be sentenced in February.