Thanks you all for that information.
I have created a quick tree for this family to see if there are any hints.
There are two trees for this family - I haven't checked any of this in any detail yet, but these trees show the son Joseph marrying Elizabeth Smith in Manchester in 1853 - he shows his father as Charles Bennett , Gardner and the 1861 census has Joseph Bennett born Spondon living in Chorlton upon Medlock, Lancashire, so this seems plausible. He goes on to die in Fairfield, Derbyshire in 1867 with children born in both counties.
The trees also show the Sarah Bennett buried in Spondon in 1837 as Charles' mother, and from the trees she appears to be Sarah Moss married to Joseph Bennet in Heanor in 1823. Perhaps the Joseph buried in 1838 is Charles' father?
Unfortunately, neither tree shows the fate of any of the other children.
A possible sighting for Mary in 1841 - Ancestry has a Mary Bennett age 9 indexed her on her own, but the image shows her living with William Grundy, joiner and his wife Sarah in Stanley.
I will continue to look into this family, but I wonder if this Sarah is too young to be marrying in Melbourne in 1845?
EDIT: The 1871 census has Charles Bennett born Heanor, Brother-in-law living with Elizabeth Bennett, Head in Fairfield, so I think we can rule out Charles going to Australia and taking younger siblings with him.