You will need to follow the path that Yowie Gully takes. It starts in the hills of the Sutherland suburbs of Gymea, goes through Miranda and finishes in Caringbah where it ends in the waters of Port Hacking. It goes under Wandella road, Kiora road and President avenue. The gully takes storm water from the streets it passes through.
At GZ you will find an object with the path of the gully in it. Follow it and you will be able to obtain the log.
Afterwards please repair the container and hide it carefully.