Joe,
I believe the number comes from Australian Standards, which would be the basis of all Australian regulations relating to the storage of flammable material.

Bunding for 200L does not need to be a steep sided structure as long as 110% of the storage capacity can be contained within the bund.
A bund to store 200L could be 2.1M * 1M * 100mm deep, or 5M * 4.2M * 10mm average depth. The oil preparation bay of my workshop is 7M *3M and has a slope from the outside to the middle (where the sealed sump resides), of 200mm This gives a capacity of (in the order of) 210L, plus the capacity of the sump (~15 - 20 litres).

I hope this helps,
Tony