Complex sentence: Rapid pastoral expansion and an increase in the colony's population triggered Aboriginal resistance from 1824 onwards when it has been estimated by Lyndall Ryan that 1000 Aborigines remained in the settled districts.
Split into simpler sentences: Rapid pastoral expansion and an increase in the colony's population triggered Aboriginal resistance from 1824 onwards. This resistance first took shape in 1824 when it has been estimated by Lyndall Ryan that 1000 Aborigines remained in the settled districts.