The Explosive Ordnance Response Team attended and deemed the device "inert".
The Guardian Angels Primary School and a neighbouring aged care centre were locked down but police concluded there was no longer any risk to the public safety.
"It appears that it's a single-shot firearm," Inspector Barry Smith told Nine yesterday.
"When it discharged, it actually deactivated itself."
Investigations are continuing.
CCTV is being reviewed from the surrounding areas to work out how the device came to be in the car park.