One and a half million cubic metres of dirt and rock sounds like a lot of dirt and rock. But even that will not be enough to fill the giant pit that has troubled the Hornsby Shire for more than a decade.

The state government has granted approval for the pit, a former quarry, to be filled with spoil from the construction of the North Connex motorway tunnel.

Hornsby Council bought the the quarry for $26 million in 2002, but discovered that making the land stable and usable would cost almost three times as much.

Residents were slugged with a 10-year rate levy to help the council pay for the quarry, and a court battle over the incorrect valuing of the site followed.