Sea Shepherd Australia's Marine Debris Campaign has removed more than 7 million items since 2016. Local chapters run monthly clean-ups; volunteers sort and count everything so the data can be reported. Events are listed on the beach cleanups news page and through chapters.
Honest caveat: dates are chapter-driven, so check the events list rather than turning up. No experience or gear needed.
How to start: seashepherd.org.au/our-campaigns/marine-debris-campaign/ volunteer at seashepherd.org.au/get-involved/