Possible extra children besides Mary Jane, born to Burn/Thurlbeck in Sunderland:
Mary Jane, M qtr 1863 Vol:10a Pg 466 mmn Thurlbeek
John George, D qtr 1864 Vol:10a Pg 497 mmn Thurlbeck
Zachariah Wilkinson, S qtr 1866 Vol:10a Pg 521 mmn Thurlbeck - died 1867
Isabel, J qtr 1868 Vol:10a Pg 581 mmn Thurlbeck
Possible Isabel and her mother in 1871

1871 Census Sunderland Union Workhouse, Bishopwearmouth
Christiana Burns,29, married, born Sunderland
Isabella Burns,2, born Sunderland
So possibly the family didn't follow Zachariah to Australia.