In the early 1970s, Croll became one of the 13 women environmental activists known as "
The Battlers for Kelly's Bush", who campaigned against urban development into a parcel of native
bushland by the
Parramatta River in
Hunters Hill, a suburb west of Sydney. Their efforts led to the first enforcement of a
Green Ban. This successful preservation of Kelly's Bush paved the way for a number of similar Green-Ban actions, impacting land use and redevelopment plans in and around Sydney. With no further real estate developments permitted on site, Kelly's Bush was later purchased by the government to become public open space, to be cared for by local volunteers known as "Friends of Kelly's Bush", with Croll as the inaugural president in 1996. She later remarked that her participation in saving Kelly's Bush as "the most important thing I ever did." == Career in medicine ==