Is there scope for a mini wetland or detention pond? Retaining stormwater runoff on your site makes a lot of sense. Although the common edict is to "hasten water on its way" perhaps there is scope in your design for the reverse? As one of the driest continents on earth, doesn't it make sense to hoard as much water on site as possible?

Remember that many of our sub catchments suffer significant impacts from urban stormwater flows. The plight of streams suffering the effects of contamination has been well publicised. However, the velocity (or speed) of the flow, plus the volume of water being discharged into the creek system in a short space of time also creates significant impacts.

The more pervious surfaces in your plan the better. These allow on-site infiltration of water that would otherwise be lost to your garden. Consider using alternatives to concrete and paving. Think about ways to catch and detain water on site for use by your garden plants rather than allowing it to speed away.

Use surface materials that maximise water absorption and reduce surface runoff. The ideal surface is vegetated and mulched with a deep organic layer to develop a good porous soil structure.