Police believe the seized firearm, a long-barrelled Breda Brescia semi-automatic shotgun with detailed engravings, may have been stolen and would like to locate its legitimate owner.
It has a serial number of 14326 and has Sportco printed on the base of the stock.
Anyone who has information on the original owner of this firearm is asked to contact police on 105 and quote file number 201220/8063.
Two women, 35 and 24, have been charged with murdering Tongia, while a 20-year-old woman has been charged with being an accessory after the fact. The 24-year-old woman is also accused of assaulting Tongia with a knife earlier in the night.
Court documents allege the younger woman "actively suppressed evidence" by hiding a firearm at an associate's property to help someone avoid conviction.
All three women have been remanded in custody.
Detective Inspector Darrell Harpur earlier said the main alleged offenders have now been charged, but he was confident they wouldn't be the last.
"I want to acknowledge the investigation team who worked tirelessly on this very complex case, and commend them on their professionalism and dedication to holding those involved to account. They worked long hours and took valuable time away from family throughout the holiday period to ensure justice was done for Rau Tongia."