Will Perrett records that Harriet is the wife of Edward STYLES and they and their children Sarah, Emily, George and Henry arrived on board the "Neptune" on 26 September 1839 as assisted migrants.
There these birth registrations in NSW that may be relevant:
STYLES JAMES 449/1849 V1849449 155 EDWARD HARRIET
STYLES JAMES 1930/1849 V18491930 55 EDWARD HARRIETT
However Mary was born in 1856 whilst the Harriett above was born about 1804. This would mean that Harriett was 54 years of age when Mary was born.