A cache of illegal guns has been seized by state, federal and US authorities as part of an investigation to tackle an international weapons operation.

Investigations began after an armed robbery at a western suburbs McDonald's 15 months ago, where a Thureon AR-15 assault rifle was used.

That weapon was then "backtracked" by police, which helped prevent a large consignment of weapons being imported to Australia.

In June, US officials seized six fully automatic assault rifles and 96 semi-automatic handgun frames, which police allege were due to be imported to Australia.