Police have warned of the threat from a plastic gun that "will kill at both ends".

The gun only costs $35 to make on a 3D printer, with instructions downloaded from the internet.

NSW Police Commissioner Andrew Scipione said on Friday police easily made two Liberator plastic guns on a $1700 home 3D printer.

From 16 parts it only took police 27 hours to build the guns. Despite the how-to guide being removed from the web earlier this month at the request of the US government, it has already been downloaded 100,000 times across the world and remains available on file-sharing networks.