New South Wales lawmaker David Elliott referred his own case to police after photos emerged showing him with the guns at a 2018 opening for the range.
Australia bans civilian use of semi-automatic and automatic weapons but exceptions can be made with permission.
Both Mr Elliott and prison officials blamed it on an "administrative error".
Australian media reported he fired a submachine gun and a semi-automatic pistol at the newly refurbished range in Sydney in September 2018.
In NSW, someone who uses an unauthorised firearm can be jailed for up to 14 years.
Read more: https://www.bbc.com/news/world-australia-51811638
