Why is the Total Volume Calculated?
The impervious surfaces of your home such as your roof and driveway are known as the catchment areas. Most councils provide a calculation based on historical local rainfall that is used to ensure you have adequate drainage for your home during storms. Those councils that do not specify use the 1 in 20 year event for calculations. Some council's require far more volume for a variety of reasons.
The idea behind the calculation is that based on the size of an impervious area (roof) and the predicted rainfall of a locality you can determine the amount of water runoff that will occur in a certain amount of time. This ensures a methodical approach and allows you to be confident you have a well designed stormwater system, that won't cause you any issues in the future.
The key point is that regulations are there to protect you. If there is no specific regulation in your area use the standard 1 in 20 calculation. Even if you decide to provide less volume at least you are making an informed decision. The cost of repairing drainage down the track far outweighs the marginal difference in the price to do it properly from the start.