Armstrong descendants from Cornwall, England

Hi, I'm trying to trace my great,great grandmother's descendants/relatives. Her name was Mary Elizabeth Armstrong. She was born on 29/03/1853 in Cornwall and died in Innisfail, Australia on: 08/09/1951. She married William Gill Northey, who was born in Callington, UK. Together they had 19 children. Mary Elizabeth's parents were George Armstrong and Catherine Anne (nee Hampton).