My heart skipped a beat when I read your posting of Mary Light (your gg grandmother?).
My husband is descended from the Light line. We have only limited information about the Light ancestry and from the little we know, Captain Francis Light (my husband's GGGG grandfather) was born in Darlinghoo, SUffolf, Eng in 1740. He went on to found Penang, Malaysia in 1786, the year one of his sons - WIlliam was born. He was married to Martina Rozells who also bore him Mary, Sarah, Lanoon and Lukey (as listed in his will dated 1794.)
We are not sure which of the Light children my husband is descended from.
William returned to England in 1792 when he was 6 and never saw his father again. 50 years after his father founded Penang (around 1836), William Light as the first Surveyor-General of the new colony of SOuth Australia, founded the city of Adelaide.
It is interesting to me that you mentioned that Mary Light your ancestor moved to Australia in 1857. Any connection I wonder.