Western Australia Police discovered the bunker in a property in High Wycombe, Perth, last year following a tip off.
From there, a staircase led to an underground bunker containing an arsenal of high-powered weapons and even a shooting range.
There was also a workbench for Letizia to work on his weapons, suppressors for his guns, and body armour.
The bunker stored a variety of different calibre bullets, including boxes of 50-calibre rounds for one of his rifles.
Letizi was charged with possession of unlicensed ammunition, possession of unlicensed firearm, possession of bulletproof clothing and inadequate storage of ammunition at home. He was fined $3,200 AUD ($2,213/£1,835), according to news.co.au.
He will also face court later this year charged with illegally building the bunker without the approval of the local council.
The bust came shortly before the biggest shake-up of Western Australia's gun laws in 50 years, and police minister Paul Papalia said the state's current laws meant Letizia escaped serious penalties.