To help answer this question, the EPA have recently released forty bottles containing GPS trackers into NSW waterways, including the Cooks River. The study will demonstrate how far litter can travel and help educate people about the damage it is doing to the environment.
We know from clean-ups on the Cooks River that plastic bottles account for the top item found and is regularly visible as pollution in the river. One of the bottles released in the Cooks River has even travelled as far as into and across Botany Bay, and back up Georges River.
The 40 bottles will be retrieved at the end of the study. In the meantime however, their progress can be tracked at https://www.facebook.com/heytosser1