Detective Sergeant Tony Langer said police allegedly found counterfeit $50 notes after a vehicle stop in Mandurah on November 13.
He said the fake money was of a reasonable quality, so people should check their money carefully and report any irregularities.
A replica gun, knives, masks and 8g of methylamphetamine were allegedly found in the vehicle.
Police claim a subsequent search of a Silver Sands home uncovered unsecured fire-arms and a crossbow.
A Silver Sands man was charged with possessing a prohibited drug with intent to sell and/or supply, two counts of possessing a prohibited wea-pon and unlawful possession. He is due in Mandurah Magistrates Court on December 13.