If you live in Sydney Australia then your house is not designed for rain. Your house will be hit by heavy rain and will flood but the government does not require house builders to allow for rain.

If you have a home on unlevel land then you are probably on one of Sydney's wet hilly areas. You are receiving downpours over 500 times more than the average rainfall you house is designed to handle.

There is token rain collection and drainage in Sydney homes but not nearly enough for Sydney. Sydney has big flat lowland dry areas and small high rainy areas. The minimum standards for building houses are designed to cope with only the trivial rain in the dry areas.