This is a local tree planting event as part of National Tree Day, which the official website calls Australia's largest tree planting and nature care event. The Point Waylen site states that the planting takes place on the foreshore reserve in Attadale along the Swan River.
Why it's worth attending: a good community/sustainability event with a clear public benefit; plus, the entire initiative has a strong reputation—the organizers point out that since 1996, 5 million volunteers have participated, 28 million trees have been planted, and 10 million hours have been donated.
Fee or not: Free, but registration is required for participants/volunteers. The official pages specifically refer to volunteer participation, not ticket purchases.