4.5.7 On-site Disposal

This refers to the discharge of stormwater on site as opposed to "on-site mitigation" which refers to various on-site measures used to mitigate effects such as the volume and rate of stormwater runoff from the site. When on-site mitigation is implemented an approved stormwater outfall is still required for the final discharge of the stormwater.

On-site disposal of stormwater is generally not an acceptable method for stormwater disposal due to the underlying clay soils, typically steep terrain and thin layer of topsoil as it may result in excessive overland flow onto neighbouring properties. There are, however, some situations in which on-site disposal may be permitted:

1. When the site is located in an area identified as being suitable for on-site ground soakage of stormwater, and on-site infiltration tests confirm this. This is generally limited to some areas around Devonport in North Shore City. Refer also to Section 4.5.1.
2. When the site borders a stream and there are no downstream properties that could be affected by runoff from the site.
3. When the site borders a coastal area and there are no downstream properties that could be affected by runoff from the site.
4. In rural areas where property sizes are large enough to accommodate the additional stormwater runoff and where this will not adversely affect neighbouring properties. In such cases stormwater must be mitigated on site through the use of measures such as rainwater harvesting and peak flow attenuation. Overflows shall disperse to land provided that the receiving area is equal to or larger than the area being drained.
5. For small impervious areas less than 25m2 provided that the receiving area is at least four times the size of the area being drained and that this will not adversely affect neighbouring properties.

Under exceptional circumstances, situations not meeting one or more of the above criteria may be considered, subject to approval by the Council. In such situations, as a minimum, the stormwater runoff from the site shall be reduced to pre-development levels through the use of measures such as rainwater harvesting, detention and bioretention (rain gardens).

Overland flow paths shall be provided in accordance with Section 4.3.2 to cater for events exceeding the capacity of the primary system and occasions when the primary drainage system fails.