The regulations for above-ground fuel storage on farms (greater than 4ha) require a containment bund be used where more than 2000 L of petrol or diesel is stored in any one fuel tank, or there is risk of a spillage endangering any building or waterway. For other applications of fuel storage (not farms), bunding is required where up to 1000 L or more of fuel is stored. Requirements are dependent on the location and local certifier. Your local authority should be able to advise of the requirements for single skin tanks.
Mainline manufactures containment bunds for tanks up to and including 5000 L are manufactured from 2.5mm galvanised sheet. The 10,000 L bund is manufactured from 3mm galvanised sheet.