Senior Constable Peter Williams said the fuel tank was also fitted with a bowser, hose, and pump to distribute the stolen fuel.
Police also located and seized two gel blasters from the property.
Willoughby and the woman were arrested and charged with multiple counts of making off without payment, possessing a firearm without a license and breach of bail.
Willoughby has been charged with petrol drive-offs from at least 19 service stations across Adelaide’s northern, eastern and western suburbs.
The pair both fronted court Monday.
Willoughby was remanded in custody and will reappear in May.
The woman was granted bail to reappear next month.