The accident happened in Berowra Heights, near Hornsby, with the woman falling around 10m while abseiling down a cliff.
NSW Police, Ambulance and Fire and Rescue were called.
Fire and Rescue NSW said on twitter they were working with paramedics to help a lady who had fallen in bushland near Wideview Rd.
Police said offers from Ku-ring-gai Police Area Command were called at 1pm following reports a woman was seriously injured after falling while abseiling.
“Emergency services treated her at the scene, however, she was unable to be revived,” they said in a statement.
“Three other people, two women and a man, were rescued off a rock ledge and brought to safety by officers from the Rescue and Bomb Disposal Unit and Fire and Rescue NSW.”
Police said they are investigating.