Tactical Response Group officers stormed a Yanchep property last night after firefighters reported a man with a gun tucked into his pants.
Firefighters were called to the Grampians Loop house about 10.45pm, where a front yard swing set had caught fire.
While speaking to the resident, firefighters spotted what they believed to be a firearm poking out of his pants.
They immediately left the property and called police, who surrounded the house and unsuccessfully tried to communicate with the man.
TRG officers were then called in to storm the house and take the man into custody.
He was found with a replica firearm in his possession.
Inquiries are continuing.