The Catchment lies to the north of Sydney, about 40 minutes by road. It is bounded by the Hawkesbury-Nepean River to the north, and by mountain ridges that enclose it on the east, south and west. It covers an area of many thousands of hectares, mainly National Parks and Bushland Reserves, although 38,000 hectares are classified as urban areas, and are home to some 80,000 people.

The urban areas in the south and south-east are on sewerage, but rural dwellings and small townships rely on septic disposal systems.

Urban development from the 1970s until the turn of the century was not attuned to ecological sustainability, and has had a significant impact on the quality of the water in the Creek.