B. cart was recorded a few decades ago in the Black Mountain bushland not far south of Belconnen Way. Also, there was (is?) a large occurrence in the lower reaches of Oakey Creek on the north end of the Tidbinbillas. Mind you, that was in the late 1970s!
I last saw it in Aranda Bushland in 2008, where a few fronds were growing at a location where a major water pipe had a long term leak. When the pipe was repaired the fern disappeared.